de Ville; (9) 31, Rue des Bourdonnais. These two circuits have double cables, and each constitute two parallel circuits, including or neglecting the batteries of certain stations in which these are cut out by a switch. The object of this arrangement is to balance the different sub stations during normal periods, and to allow of provisional grouping in case of accidents.

The third charging circuit fed from the Rue Dieu has a single cable, and includes the following sub-stations-viz., (1) 83, Rue Turbigo; (2) 9, Rue Bourg l'Abbé ; (3) Passage Lemoine; (4) 4, Rue d'Hauteville; (5) 13, Boulevard Voltaire.

Finally, there is a fourth station working by means of steam in the Boulevard Richard-Lenoir, having two charging circuits, one of which feeds sub-stations at 6, Rue Malher and 20, Rue de la Verrerie, and the other, sub-stations at 70, Boulevard Beaumarchais and 6, Rue de Franche-Comté. These four circuits are completely separate and autonomous; at the same time there are localities where auxiliary cables, called dead cables, afford connections between neighbouring circuits, which in regular working are cut out, but which, in case of accident on any of these circuits, enable it to be supplied by one of the others. The charging current is maintained constant at 200 amperes; the pressure varies directly as the number of accumulators under charge, and may rise to 2,400 volts. There are two batteries of accumulators in each sub-station in order that the permanence of the discharge on the secondary circuit supplying customers may be assured.

done? By means of an apparatus called "the undulator, devised by M. Solignac, the object of which was to employ transformers on continuous current circuits. We saw this apparatus working at the Centenary Exhibition of 1889. It fed Jablochkoff candles by Gaulard and Gibbs transformers, with a current obtained from a continuous-current dynamo. If our memory does not deceive us, the ruling principle of this device consists in placing two transformers in the substations, putting an "undulator" in shunt with each transformer, and in the transformer branch a current reverser.

The "undulator" is a kind of rotating commutator, which places in circuit with the first transformer a series of resistances so arranged that at the moment of the short circuit, indicated at a in Fig. 2, no current passes in the in the second transformer and in the line. corresponding transformer, whilst the whole current is on

As the "undulator" rotates, so the resistances b, c, d, e, f, g, are inserted in shunt with the transformer; the current increases in the latter as shown by the curve a b, Fig. 3, up to the point of complete opening of the circuit, where it receives all the current, that is at the point b. Continued working of the apparatus again successively cuts out the

FIG. 1.-A A A A. Charging stations. BB B. Sub-stations in series. cc. Radiating circuit. d d. Customers' leads. EE. Lamp circuits. LLL. Charging circuits.

The secondary circuit is laid in two different ways. In the central parts of the city it consists of circuits radiating round the sub-station, about 250 metres in length, on which are placed special installations; the excess charge in the houses of the customers nearest to the station is absorbed by rheostats on their premises. All the sub-stations of the Retiro circuit, of the Bourse de Commerce, and of the Turbigo, Rue Bourg l'Abbé, and Passage Lemoine circuits, are arranged on this principle. All the other stations near the peripheral districts feed loop circuits, the two conductors, both positive and negative, returning under the footway on the opposite side of the street, so as to serve both sides. The conductors are of bare wire, placed in troughs and mounted on porcelain insulators. The charging cables, as well as those of the radiating circuits, are insulated by a coating of indiarubber, covered with a protecting envelope of lead and hemp. The first are very highly insulated; their cost reaching to 20,000f. per kilometre; the second mentioned, which are not so costly, work out to 16,000f. per kilometre.

The distributing arrangements adopted, and those which proceed therefrom, are graphically summed up in Fig. 1. The general idea has been worked out from the suggestions of M. Solignac. The mention of his name reminds us that we have to complete the exposition of the economy of the sector. It was a question of working with alternate-current transformers side by side with accumulators. How was this

FIG. 3.

resistances g, f, e, d, c, b, up to the moment when the transformer is short circuited. This point is utilised to reverse the poles of the transformer, and, as a consequence, a new curve, cd, is described below the axis, x x'. The undulator is always in duplicate, so that when one transformer is short circuited the other is not, the object being to equalise the work on the machine.

(To be continued.)

A NON-INDUCTIVE WATTMETER.

Great difficulty has always been felt in measuring power with alternate currents. The ordinary wattmeter, though accurate with direct currents when a correction is made for the power taken by the instrument itself, is of little value for use with alternate currents, as the time-constant of the fine wire circuit is appreciable. If the pressure and current are in step, as is the case when an alternate pressure is applied to a resistance, such a wattmeter reads too low. If a pressure is applied to an inductive circuit, on the other hand, the reading may be much too high.

Messrs. Swinburne and Co. have brought out a wattmeter specially designed to avoid these errors. The moving coil contains but few turns, and these are wound on a light mica former. The coil is held by top and bottom stretched wires. External resistances are supplied, wound with the alternate layers right and left handed. The time-constant of the fine wire circuit is thus made sensibly equal to zero. Readings are taken by means of a torsion head in the usual way, but for measurements of minute powers, such as hundredths of a watt, a mirror is used. These wattmeters are wound to suit any range from 3,000 volts, and from 25 amperes downwards.

They are stated by the makers to read accurately on noninductive resistances, and on inductive circuits in which the product of the pressure and current is 50 times the real power.

The wattmeter has the advantage over electrometer methods, that it does not involve the very serious troubles over resistances. Mr. Swinburne recently described an electrometer which reads power directly, which, though it does the work of the three voltmeters discussed by Messrs. Ayrton, Sumpner, and Swinburne soon after, unfortunately

requires resistances. Dr. Fleming has enormously improved the three-instrument method by substituting amperemeters, and we thus have unimpeachable means of verifying the wattmeter. If this wattmeter does what the makers claim for it, it will be exceedingly valuable to alternating-current engineers.

EXPERIMENTS WITH ALTERNATE CURRENTS OF VERY HIGH FREQUENCY AND THEIR APPLICATION TO METHODS OF ARTIFICIAL ILLUMINATION.*

the globe, and although the exhaustion may be carried to the highest degree, still the space inside of the bulb must be considered as conducting when such high potentials are used, and I assume that in estimating the energy that may be given off from the filament to the surroundings we may consider the inside surface of the bulb as one coating of a condenser, the air and other objects surrounding the bulb forming the other coating. When the alternations are very low there is no doubt that a considerable portion of the energy is given off by the electrification of the surrounding

air.

In order to study this subject better, I carried on some experiments with excessively high potentials and low frequencies. I then observed that when the hand is approached to the bulb-the filament being connected with one terminal of the coil-a powerful vibration is felt, being due to the attraction and repulsion of the molecules of the air which are electrified by induction through the glass. In some cases where the action is very intense I have been able to hear a sound, which must be due to the same cause.

When the alternations are low, one is apt to get an excessively powerful shock from the bulb. In general, when one attaches bulbs or objects of some size to the terminals of the coil, one should look out for the rise of potential, for it may happen that by merely connecting a bulb or plate to the terminal, the potential may rise to many times its original value. When lamps are attached to the bulbs should be such as to give the maximum rise of potenterminals, as illustrated in Fig. 24, then the capacity of the tial under the existing conditions. In this manner one tial under the existing conditions. may obtain the required potential with fewer turns of wire. course, largely on the degree of exhaustion, but to some The life of such lamps as described above depends, of extent also on the shape of the block of refractory

BY NIKOLA TESLA.

(Continued from page 131.)

With these rapidly alternating potentials there is, however, no necessity of enclosing two blocks in a globe, but a single block, as in Fig. 20, or filament, Fig. 23, may be used. The potential in this case must of course be higher, but it is easily obtainable, and besides it is not necessarily dangerous.

The facility with which the button or filament in such a lamp is brought to incandescence, other things being equal,

FIG. 23.

depends on the size of the globe. If a perfect vacuum could be obtained, the size of the globe would not be of importance, for then the heating would be wholly due to the surging of the charges, and all the energy would be given off to the surroundings by radiation. But this can never occur in practice. There is always some gas left in * Lecture delivered before the American Institute of Electrical Engineers at Columbia College, New York, May 20.

FIG. 24.

material. Theoretically it would seem that a small sphere of carbon enclosed in a sphere of glass would not suffer deterioration from molecular bombardment, for, the matter in the globe being radiant, the molecules would move in straight lines, and would seldom strike the sphere obliquely. An interesting thought in connection with such a lamp is, that in it "electricity" and electrical energy apparently must move in the same lines.

The use of alternating currents of very high frequency makes it possible to transfer, by electrostatic or electromagnetic induction through the glass of a lamp, sufficient energy to keep a filament at incandescence and so do away with the leading-in wires. Such lamps have been proposed, but for want of proper apparatus they have not been successfully operated. Many forms of lamps on this principle, with continuous and broken filaments, have been constructed by me and experimented upon. When using a secondary enclosed within the lamp, a condenser is advantageously combined with the secondary. When the transference is effected by electrostatic induction, the potentials used are, of course, very high with frequencies obtainable from a machine. For instance, with a condenser surface of 40 centimetres square, which is not impracticably large, and with glass of good quality 1 mm. thick, using currents alternating 20,000 times a second, the potential required is approximately 9,000 volts. This may seem large, but since each lamp may be included in the secondary of a transformer of very small dimensions, it would not be inconvenient, and, moreover, it would not produce fatal injury. The transformers would all be preferably in series. The regulation would offer no difficulties, as with currents

of such frequencies it is very easy to maintain a constant

current.

In the accompanying engravings some of the types of lamps of this kind are shown. Fig. 25 is such a lamp with a broken filament, and Fig. 26A and Fig. 26B one with a single outside and inside coating and a single filament. I have also made lamps with two outside and inside coatings, and a continuous loop connecting the latter. Such lamps have been operated by me with current impulses of the enormous frequencies obtainable by the disruptive discharge of condensers.

The disruptive discharge of a condenser is especially suited for operating such lamps-with no outward electrical connections by means of electromagnetic induction, the electromagnetic inductive effects being excessively high;

ប

FIG. 25.

necting one terminal of the lamp to one terminal of the source, and the other to an insulated body of the required size. In all cases the insulated body serves to give off the energy into the surrounding space, and is equivalent to a return wire. Obviously, in the two last-named cases, instead of connecting the wires to an insulated body, connections may be made to the ground.

The experiments which will prove most suggestive and of most interest to the investigator are propably those performed with exhausted tubes. As might be anticipated, a source of such rapidly alternating potentials is capable of exciting the tubes at a considerable distance, and the light effects produced are remarkable.

During my investigations in this line I endeavoured to excite tubes, devoid of any electrodes, by electromagnetic induction, making the tube the secondary of the induction device, and passing through the primary the discharges of a Leyden jar. These tubes were made of many shapes, and I was able to obtain luminous effects which I then thought were due wholly to electromagnetic induction. But on carefully investigating the phenomena I found that the effects produced were more of an electrostatic nature. It may be attributed to this circumstance that this mode of exciting tubes is very wasteful-namely, the primary circuit being closed, the potential, and consequently the electrostatic inductive effect is much diminished.

FIG. 26B.

FIG. 26A.

and I have been able to produce the desired incandescence with only a few short turns of wire. Incandescence may also be produced in this manner in a simple closed filament. Leaving, now, out of consideration the practicability of such lamps, I would only say that they possess a beautiful and desirable feature-namely, that they can be rendered at will more or less brilliant, simply by altering the relative position of the outside and inside condenser coatings, or inducing and induced circuits.

FIG. 28.

When an induction coil, operated as above described, is used, there is no doubt that the tubes are excited by electrostatic induction, and that electromagnetic induction has little, if anything, to do with the phenomena.

This is evident from many experiments. For instance, if a tube be taken in one hand, the observer being near the coil, it is brilliantly lighted and remains so in no matter what position it is held relatively to the observer's body Were the action electromagnetic, the tube could not be lighted when the observer's body is interposed between it and the coil, or at least its luminosity should be considerably diminished. When the tube is held exactly over the centre of the coilthe latter being wound in sections and the primary placed

When a lamp is lighted by connecting it to one terminal only of the source, this may be facilitated by providing the globe with an outside condenser coating, which serves at the same time as a reflector, and connecting this to an insulated body of some size. Lamps of this kind are illustrated in Fig. 27 and Fig. 28. Fig. 29 shows the plan of connections. The brilliancy of the lamp may in this case be regulated within wide limits by varying the size of the insulated metal plate to which the coating is connected.

It is likewise practicable to light with one leading wire lamps, such as illustrated in Fig. 21 and Fig. 22, by con

FIG. 29.

symmetrically to the secondary-it may remain completely dark, whereas it is rendered intensely luminous by moving it slightly to the right or left from the centre of the coil. It does not light because in the middle both halves of the coil neutralise each other, and the electric potential is zero. If the action were electromagnetic, the tube should light best in the plane through the centre of the coil, since the electromagnetic effect there should be a maximum. When an arc is established between the terminals, the tubes and lamps in the vicinity of the coil go out, but light up again when the arc is broken, on account of the rise of potential. Yet the electromagnetic effect should be practically the same in both cases.

By placing a tube at some distance from the coil, and nearer to one terminal-preferably at a point on the axis

of the coil-one may light it by touching the remote terminal with an insulated body of some size or with the hand, thereby raising the potential at that terminal nearer to the tube. If the tube is shifted nearer to the coil so

that it is lighted by the action of the nearer terminal, it may be made to go out by holding, on an insulated support, the end of a wire connected to the remote terminal, in the vicinity of the nearer terminal, by this means counteracting the action of the latter upon the tube. These effects are evidently electrostatic. Likewise, when a tube is placed at a considerable distance from the coil, the observer may, standing upon an insulated support, between coil and tube, light the latter by approaching the hand to it; or he may even render it luminous by simply stepping between it and the coil. This would be impossible with electromagnetic induction, for the body of the observer would act as a screen.

When the coil is energised by excessively weak currents, the experimenter may, by touching one terminal of the coil with the tube, extinguish the latter, and may again light it by bringing it out of contact with the terminal and allowing a small arc to form. This is clearly due to the respective lowering and raising of the potential at that terminal In the above experiment, when the tube is lighted through a small arc, it may go out when the arc is broken, because the electrostatic inductive effect alone is too weak, though the potential may be much higher; but when the arc is established, the electrification of the end of the tube is much greater, and it consequently lights.

If a tube is lighted by holding it near to the coil, and in the hand which is remote, by grasping the tube anywhere with the other hand, the part between the hands is rendered dark, and the singular effect of wiping out the light of the tube may be produced by passing the hand quickly along the tube and at the same time withdrawing it gently from the coil, judging properly the distance so that the tube remains dark afterwards.

If the primary coil is placed sidewise as in Fig. 17B for instance, and an exhausted tube be introduced from the other side in the hollow space, the tube is lighted most intensely because of the increased condenser action, and in this position the stria are most sharply defined. In all In all these experiments described, and in many others, the action is clearly electrostatic.

The effects of screening also indicate the electrostatic nature of the phenomena and show something of the nature of electrification through the air. For instance, if a tube be placed in the direction of the axis of the coil, and an insulated metal plate be interposed, the tube will generally increase in brilliancy, or if it be too far from the coil to light, it may even be rendered luminous by interposing an insulated metal plate. The magnitude of the effects depends to some extent on the size of the plate. But if the metal plate be connected by a wire to the ground, its interposition will always make the tube go out, even if it be very near the coil. In general, the interposition of a body between the coil and tube, increases or diminishes the brilliancy of the tube, or its facility to light up, according to whether it increases or diminishes the electrification. When experimenting with an insulated plate, the plate should not be taken too large, else it will generally produce a weakening effect by reason of its great facility for giving off energy to the surroundings.

If a tube be lighted at some distance from the coil, and a plate of hard rubber or other insulating substance be interposed, the tube may be made to go out. The interposition of the dielectric in this case only slightly increases the inductive effect, but diminishes considerably the electrification through the air.

In all the cases, then, when we excite luminosity in exhausted tubes by means of such a coil, the effect is due to the rapidly alternating electrostatic potential; and, further. more, it must be attributed to the harmonic alternation produced directly by the machine, and not to any superimposed vibration which might be thought to exist. Such superimposed vibrations are impossible when we work with an alternate-current machine. If a spring be gradually tightened and released, it does not perform independent vibrations; for this a sudden release is necessary. So with the alternate currents from a dynamo machine: the medium is

harmonically strained and released, this giving rise to only one kind of waves; a sudden contact or break, or a sudden giving way of the dielectric, as in the disruptive discharge of a Leyden jar, are essential for the production of superimposed waves.

In all the last described experiments, tubes devoid of any electrodes may be used, and there is no difficulty in producing by their means sufficient light to read by. The light effect is, however, considerably increased by the use of phosphorescent bodies such as yttria, uranium, glass, etc. A difficulty will be found when the phosphorescent material is used, for with these powerful effects it is carried gradually away, and it is preferable to use material in the form of a solid.

Instead of depending on induction at a distance to light the tube, the same may be provided with an externaland, if desired, also with an internal-condenser coating, and it may then be suspended anywhere in the room from a conductor connected to one terminal of the coil, and in this manner a soft illumination may be provided. (To be continued.)

NOTES ON ELECTRICAL WORK IN MINES.*

The

BY ALBION T. SNELL, ASSOC.M.I.C.E., M.I.E.E. Perhaps no branch of engineering has received more attention of late than the application of electricity to the ordinary purposes of mining. At home, on the Continent, and in America, electrical energy has been applied to this work, and in the majority of instances with success. general theory underlying the application of this power has been discussed again and again, and numerous illustrations have from time to time been given, so that in reopening the matter before the South Wales Institute of Engineers, I feel some difficulty in selecting the precise lines for my paper. The theoretical side of the question is not a suitable subject for discussion here, and from the practical point of view, plants illustrative of pumping, hauling, coal cutting, etc., have been shown so often as to lessen the immediate interest. My object in reading this paper, then, is simply to open a discussion to induce members to give their various experiences of electric work, so that by a comparison of such data we may be enabled to form an adequate idea of the work now being performed in collieries by this agency, and I hope that there will ensue a lengthy discussion which will materially add to the practical knowledge now extant on this important department of engineering.

With this view I purpose, even at the risk of describing what has already been published elsewhere, to briefly refer to one or two of the more important plants now running in Great Britain, and especially a pumping plant at Messrs. Crawshay Brothers' Newbridge Rhondda Colliery. These plants have been erected under my own superintendence and I have selected them because they are applicable for lighting as well as for power, and are so installed that the energy may be divided into separate units at different parts of the circuit, just as several engines can be driven off a compressed air system.

In February, 1890, the question of "The Electrical Distribution of Energy over Extended Areas in Mines" was discussed before the Federated Institute of Mining Engineers, and I then proposed the use of over-compounded dynamos for this purpose. When the matter was also discussed at the Newcastle meeting in 1891, I was able to state that several installations designed on these lines had been working successfully for some time. One of these was the Newbridge Rhondda Colliery pumping plant, and by the courtesy of Mr. Abraham, the manager, I am able to bring the following details before you. This plant is typical of the system; it is successful here, and there is no reason to prevent it being equally successful under similar conditions in other pits. The problem was to drive two pumps off the same set of mains. The pumps were 700 yards apart, and it was required to make each independent, so as to be stopped or started without affecting * Paper taken as read at the annual meeting of the South Wales Institute of Engineers, July 27, 1891.

the speed of the other. It was also imperative that the electric light should be used at the pumping stations, at the pit bank and pit bottom; and, further, there was to be only one dynamo. These conditions assumed that the pressure would be practically constant throughout the circuit. In practice, it is impossible to secure an absolutely constant pressure all over the system, owing to the resistance of the mains themselves; but by over-compounding the dynamo, so as to give a constant pressure at a point near the centre of the system, it is possible to approximate very closely to the required conditions. How well the case has been met is apparent when I mention that both the pumps can be switched off without appreciably raising the voltage of the lamps. The dynamo is one of our Sh. D. C. 12/16 type, and running at 800 revolutions per minute gives a pressure of 300 volts and 60 amperes. Each of the motors gives 7 b.h.p., and runs at 800 revolutions per minute. The lamps are coupled in series of three. The pumps are three-throw rams, 4in. in diameter by 9in. stroke. Mr. Abraham tells me the plant has been perfectly successful from the first, and he hopes to extend the use of electricity at an early date.

Another interesting application of electricity which is likely to be of use in certain cases is a new system of haulage patented by Messrs. G. B. Walker, of Wharncliffe Silkstone, and Immisch, of the G. E. P. and T. Company. In this system the loco does not depend for grip on its own weight, but gets a direct pull on a cable lying between the rails, parallel to the road, and fixed at either end. This cable passes over a sprocket wheel on the loco or trolley, and is driven through suitable gearing by an electric motor. The motor is supplied with energy from a bare copper wire arranged on the roof or the side of the road. It is needless to say that such a trolley is only applicable to roads free from gas. The chief advantage of the system lies in the light weight of the trolley in proportion to the heavy tractive effort it can exert. It will be principally used as an auxiliary power on short steep lengths of otherwise fairly level roads, where horses are generally used, and will be found in many cases to be a great saving in horseflesh. A loco or trolley of this type has been running at Wharncliffe Silkstone Collieries for nearly 12 months on a brow 500 yards long, with a grade averaging 1 in 9. It raises loads of about five tons at a speed of three miles per hour. Formerly, 12 horses were required for the work, and even with this number it was so severe that it was not deemed advisable to keep the same animals at it for more than six months at a time. Mr. Allison, my assistant, who erected the line, gives me the following figures, roughly measured, which will be interesting from the electrical point of view Rolling load Average speed Average grade Grade 1 in 12 Grade 1 in 11

5 corves 2.65 miles per hour.

4 tons.

1 in 91

Amperes, 45

Amperes, 50

Grade 1 in 10

Amperes, 55

Grade 1 in 9

Amperes, 65

Amperes, 70

Volts, 180. Volts, 190. Volts, 195. Volts, 200. Volts, 205.

Grade 1 in 8

Since the above figures were taken, a larger dynamo has been installed, and the loco will now do more work.

The loco trolley is unnecessarily heavy for the particular work, and was adapted for existing material; considerable improvements can be made in the design in future. The motor is rated at 10 brake horse-power.

Another interesting application of the compound parallel system is the installation at Andrew's House Pit, Durham, By the courtesy of Mr. Cuthbert Berkley I am able to give the following details: The plant comprises a dynamo, three motors, cables, and three dip pumps. The pumps are respectively 1,500, 1,800, and 2,000 yards in bye; the one nearest the shaft is driven by a 4-h.p. motor, the others each work by motors of 2-h.p. The dynamo gives 250 volts and 40 amperes. The total capacity of the plant is thus small, but it is interesting on account of the distance between the pumps and the length of the cable. A few lamps are run at each of the pump stations and also in the engine-house. The dynamo is arranged to compound for constant potential at about 1,500 yards in bye. But owing to the length of cable the pressure is not maintained so

uniformly as at Newbridge Rhondda installation. Each of these motors has replaced a crank which required 12 horses to keep it running through the 24 hours. The plant has now been at work for nearly two years. I may mention in passing that a wire rope is used for the return, and no difficulty has been experienced from this; but if the pressure were higher, or the work larger, I should advise an insulated return.

In Bohemia, my company erected last year two plants of some importance in the history of mining in that district, they being the first application of electricity to mining work there. The first is at the Standard Tin Company's St. Mauritius Mine. The dynamo is placed below ground and is driven by a turbine, the water being led down a shaft by suitable pipes. The motor is arranged to work a pair of lift pumps with spears 40 metres long, and also to wind ore through the same vertical distance. When the installation was first made the lower levels were drowned out and the electric plant had to pump them dry before work could be resumed. This was successfuly accomplished in a shorter time than the engineers expected, and ore is now being regularly raised by the plant. The power of the motor is about 20 horse. The second instance is at Eger, at one of the brown coal mines; here the main ventilation is produced by a fan driven by electricity, and it is, I believe, the only mine where this is the case. The dynamo is driven by a turbine about half a mile away from the fan house, and the current is carried by bare wires mounted on poles with fluid insulators. The power of the motor is approximately 25 horse. The fan has now been running for about 12 months, and the manager reports that it is working perfectly satisfactorily.

We are now building a very interesting plant which illustrates the conversion of water power into electrical energy and its utilisation for light and power. One thousand yards from the Greenside Lead Mines, near Ullswater, there is a waterfall of upwards of 100 h.p. A turbine running at 1,000 revolutions per minute is coupled by a belt to one of our 100 e.h.p. four-pole transmission dynamos, wound in compound to give 625 volts and about 900 amperes. The energy is transmitted by a bare cable (19/15 S.W.G.) carried on poles to the mine mouth, and there lead-covered cables are used to convey the electricity to the various motors. These will at once run a winding plant of 10 b.h p., a pumping plant of 10 b.h.p., and another winding plant of 20 b.hp. is under consideration. Electric light will be used at different points of the line, and other motors will soon be installed for various purposes. Other examples might be given, but the preceding cases are fairly representative of the work now being done.

Every system of transmitting energy must necessarily entail certain risks peculiar to itself; therefore it is not surprising that we find such difficulties in connection with the use of electricity in mines. There is no doubt that the limits of these difficulties are very imperfectly understood by many mining engineers, and a few remarks on the subject may not be uninteresting. There are two distinct difficulties to contend with which may or may not be dangerous. The first is the risk from shocks. These may be received from any part of the circuit, if the installation be faulty, and under any circumstances from the brushes, commutator, and terminals of the dynamo and motors. The danger to be apprehended from electricity on this account depends primarily on the pressure of the circuit. Opinions are very much divided as to the number of volts necessary to cause death; but as, owing to difficulties of installation, the pressure in mines is usually limited to 500 volts or thereabouts, there is no fear of a fatal accident under any ordinary conditions. I have installed many plants of about this pressure, and have never heard of a single injury occurring to either the attendants or the miners, although shocks have necessarily been frequent. At St. John's Colliery, Normanton, we have been working for rather more than three years with a maximum pressure of 700 volts, without a single accident happening from this cause; and further, no casualty from electrical shock in any other mine in England has to my knowledge been heard of; consequently, I venture to think that this danger is practically negligible. The second