that a pressure of 10,000 volts will give a spark between terminals of in. or so in air, and on a damp day sparks with the sound of a pistol-shot will jump over an inch or more of insulating surface. It was to conduct a large current at this pressure, and to secure permanently good results, that the Ferranti mains had to accomplish,

After many and exhaustive experiments the insulating bodies which were finally selected were paper and black ozokerite, or earth wax. It will be interesting here to give Mr. Ferranti's ideas upon the subject of insulation of hightension mains, a subject to which he has naturally given more than ordinary attention.

The necessary points to consider are two: durability of substance and the factor of safety. No one can definitely tell how the insulation may change in course of time, but what can be at once tested is the factor of safety of such insulation. No fault will develop in the insulation unless the dielectric is sufficiently strained to produce enough mechanical effect to cause a gradual change; in other words, unless it is strained beyond the limit of elasticity. To take an example in mechanical engineering, a connectingrod, for instance, the metal may be strained to any point within its limit of elasticity, and nothing gives way; strain it outside this limit and it may go at any time-in a week, a day, an hour, according to the overstrain. Its strength will depend upon the margin of safety. It is the same with insulation. Strained below the limit, the insulation will not give way; strained above its limit, the breakage is but a question of time. The Ferranti mains are made with a very large margin of safety, a margin that has been accurately determined by direct experiment. From these experiments it is found that a thickness of in. of paper saturated with the black wax, is pierced within one hour by 20,000 volts-some specimens will go within ten minutes, and nearly all within the hour: above that thickness the insulation is not pierced. This being so, with the present mains, which have in. of insulation, there is eight times that thickness, and with half the number of volts we have 16 as the factor of safety. In the Ferranti mains there are 60 layers of waxed paper wrapped one above the other, and the factor of safety is so large, even if one or two layers were partially faulty, it may be trusted to the remainder to give perfect safety. In point of fact, no failures of tested mains have yet been experienced by reason of direct failure of the insulation. Fifteen faults in all were experienced in the 30 miles already laid, mostly of want of continuity. Only two Only two faults were found with 20,000 volts on the whole run, both of these due to water in the joint at the time of making. Mains constructed for only 2,500 volts have been tested for months with 25,000 volts without giving way, and it is on a series of practical experiments such as these that Mr. Ferranti is relying for the safety of his larger mains. There is, we may say, a gradual mechanical change if the insulation is too thin. There is no change at all if the insulation is sufficiently thick. The pressures in the present form of mains have been doubled and trebled, running up to 28,000 volts to try and break down the insu lation, but they have been unable to do this, and so feel perfectly happy with their 10,000 volts.

Nothing but time, of course, can prove the question of durability of the insulation, but the materials have been. chosen as the most durable it is possible to find. The wax used for insulating is the very same black wax as that in which the mummies were boiled to preserve them in old Egyptian times. The oldest mummy in the British Muscum was found wrapped in linen and boiled in this same black wax. The chemically pure paper used is hermetically sealed down from the air by the tube which is drawn tightly over it. Paper, again, is one of the most permanent substances known. There is a papyrus book in existence known to have existed 3,000 years. Two of the most permanent materials in the world have therefore been used in the Ferranti insulation. It is not maintained that these mains will last 3,000 years, or 1,000, but Mr. Ferranti does consider that the chances are so enormously in their favour that they are commercially good enough for our day.

With regard to the jointing, this is a most important part of the whole system. The mains are constructed ready

for laying in the ground; the weight is heavy and the length is kept to 20ft., so that a few men can handle them. In the 30 miles of mains there are therefore seven or eight thousand joints. To allow this the joint must be abso lutely safe when once properly made. This indeed is what renders the whole scheme practicable. To make such a joint in the insulation was an achievement long doubted, but is now proved perfectly possible. The desiderata were a very long jointing surface and perfect contact. These were achieved by making a long slanting cone-like surface, the protruding point of one main fitting into the hollow of the next, these surfaces being accurately and truly turned and polished to gauge, and then heated and forced together by hydraulic pressure, by which means the insulation becomes practically one solid piece.

Having said so much as to the principle of the Ferranti mains, we will briefly again describe the manufacture for the purpose of completing this series of articles. Tubes of high conductivity copper are taken, cut into 20ft. lengths, and straightened. The usual size of tube to carry up to 250 amperes is of in. section, and the size in. inside diameter, and 18in. outside diameter. Lengths, 20ft. long, of brown paper are cut off from a roll 3ft. wide, and a length of this paper is glued by its edge to to the copper tube. Meanwhile, other rolls of paper are passed over long iron plates heated below by open fires, and thus thoroughly dried; these are passed through a bath of hot melted black mineral wax, drawn over rollers and through the air for some distance until dry; they are then cut into 20ft. sheets and placed for use on shelves. The copper tube has squared pieces of wood knocked into its ends, and is then placed in sockets of a slowly-revolving roller on a table which has at the back a set of rollers, a bath of hot wax, and revolving gear. As the tube revolves, lengths of the prepared paper are inserted between it and the brown paper, sheet after sheet, until 60 sheets are served in. During this time heavy rollers come down upon the tube, compressing the paper, and at the same time by displacement boxes dipping into the bath, the wax is made to flow up and saturate the sheets. When the required thickness is served the wax is made to flow back, the insu lation compressed still more upon the copper tube, and a tape is wound spirally over the whole. The tube covered with its insulation is then removed, the wooden pieces knocked out, and the whole slipped into a second tube of copper. This tube is of the same total cross-section as the first-viz., țin.-but being larger in diameter is proportionately thinner. The size of this tube is 13in. inner diameter, and 118in. outer diameter. The tube is left a little larger, so as to slip easily over the inner tube with its insulation, and is then passed through a die and drawn down upon the insulation. This outer tube is now served with the insulation in the same manner as the inner tube: first a length of brown paper glued by one edge, then several sheets of waxed paper to the thickness of in., compressed and taped as before. The whole is slipped into an outer iron tube to act as a protecting shield. wax is forced by a pump through a small hole in the centre beneath this iron casing till the inner space is completely filled. The whole is then sawn off at the ends into exact 20ft. lengths. The section of the main is shown in Fig. 1.

Melted

The next process is the preparation of the joint. One end of each length is formed into a projecting cone, and the other end into a hollow cone, by means of a special hollow spindle lathe. The length is placed on this lathe, the outer iron shield is removed for a distance of 17in. from the end; 14in. of the outer insulation, E, thus exposed is then turned up, leaving bare that length of the outer copper conductor, B; 6in. of this conductor is now removed, and the insulating material between the two tubes is turned carefully down, forming a cone at the end of the cable, as shown in the illustration, Fig. 2. The interior of the inner tube is then slightly enlarged by drilling for a length of 6in. inside.

The length of main is then reversed, and the insulation, C, of the other end is coned out for a distance of 6in. to exactly the same taper, while the iron shield is removed for a length of 11in., and the outer insulation for a length of 8in., leaving the outer conductor exposed for this length. The mains are now ready for laying. Each length is

separately tested to 20,000 volts, the ends are capped to prevent dirt getting in, and are then sent out to the place required. They are jointed together on the spot, as follows (see Fig. 3): A tight-fitting copper rod, a, 12in. long, is driven into the inner tube, A, of the hollow coned end. A tight-fitting sleeve of copper, F, is driven for a distance of 8in. on the outer conductor of the main to which it is to be jointed, and this sleeve firmly gripped on by means of a special tool by three or more circular corrugations, as shown. The two cones are then inserted one within the other, the surfaces being previously warmed, and are forced together and driven home by screw clamps, a total pressure of about three tons being employed, and when still under compression the copper sleeve is firmly locked to the other outer conductor by means of circular corrugations as before described. The sleeve, F, and the outer insulation, E, are wrapped at the junction with insulating material until they become of the same external diameter as the iron tube, D, when an iron sleeve, G, 30in. long, is passed over the joint and corrugated down at both ends. In order to fill up any air space in the outer insulation, hot wax is forced in through the boss, H, of the sleeve, G, the whole being finally closed with a gas plug.

The laying of concentric mains is thus relatively a very simple matter; they are supplied ready for jointing together, and may be carted out and laid as gas-pipes are, no cement channels or specially-prepared conduits being necessary. It is usual, however, in crowded streets to lay them in a wooden trough, with wooden separating slips, the trough being filled in with pitch with an upper layer of concrete for extra protection. They may be laid under any pavement or roadway, causing a minimum of disturbance and occupying a minimum of space. As will be seen, they adapt themselves readily to laying through tunnels and subways, in which case they may rest on wall brackets. When laid in this manner they are subject to variations of temperature, to compensate for which all that is necessary is to give them at certain points a slight wave in laying. To bend a main to go round a corner an ordinary rail bender, as used on railways, is employed; a curve of 6ft. radius being made in this way with but little trouble. In bending, it is found there is no appreciable drag between the layers of insulation and conductors.

For making branch connections a special T-joint is employed; this consists mainly of a cast-iron box with suitably designed base and cover arranged to fit watertight. These joints do not, of course, appear on the road surface, being inserted in the run of the mains as required. The joint has three stuffing-boxes through which the ends of the mains are brought in, A screw bolt from the centre of the branch main connects to the inner conductor of the main itself, and the joint is wrapped with paper insulation; the outer conductors are connected with a gunmetal bridgepiece of the shape shown, Fig. 4. Street boxes are also placed in the run of the mains at distances of about 200 yards. These are iron boxes similar in principle to the Tjoint boxes, but are placed in small brick chambers, having removable covers flush with the road surface, Fig. 5. The interior is thus accessible for testing and other purposes, while the arrangement of the connections is such that the joint can be easily and quickly connected or disconnected. These joint and street boxes may be filled with rosin oil, by means of which very high insulation is insured at these points, and the full pressure of 10,000 volts may be safely used. While the above description applies more particularly to mains for parallel distribution, the system may be employed with equal advantage for series work.

ductors, the current can only return direct to the machine, where the safety fuses prevent it doing any harm. The fuses employed are also illustrated herewith. The smaller one, Fig. 6, has a 12in. break, and is used in houses for the primary circuits of transformers. The larger fuse, Fig. 7, is identical with the first, except that it is adapted for main currents, and has a multiple fuse with a 24in. break. The plugs are arranged for separating the multiple fuse wires.

The absence of necessity for channels or conduits in the Ferranti system is an item which should be taken into account, and has, further, the immense advantage of avoiding the possibility of explosions from an accumulation of sewer or lighting gas, which have occurred so frequently with both high and low tension systems throughout Europe and America. Explosions of this kind have already occurred in London. In fact, when a conduit or line of pipes is opened, the presence of gas (which is found, moreover, to impair the insulation of cables laid in that manner) is very frequently detected. Such methods are also liable, sooner or later, to dangers which arise when water is present. For instance, when bare conductors are used and water gets access to them, they are liable to be shortcircuited or injured by electrolysis. With insulated cables in pipes or conduits, the alternate presence and absence of water affects the insulation, damaging it in time, and when it is present in winter, it is liable to freeze-the ice crushing or piercing the insulation and causing partial or total short-circuitings. This occurred in London during the late winter of 1890-91.

With regard to safety the following experiments were carried out :

In the presence of representatives of the Board of Trade, the Post Office, and local authorities, a main which had been running continuously under a pressure of 10,000 volts was submitted to two engineers, who, with a cold chisel and sledge-hammer, managed, after considerable time and much deliberate labour, to cut through from the outer to the inner conductor while 200 h.p. at that tension was being transmitted through it. They did not feel the slightest shock, although they were standing on a large metal plate making earth. A similar trial has been made with a pickaxe. The mains have also been submitted to a lengthened test, after laying, with 30,000 volts, and have given no trouble. It has thus been forcibly demonstrated that this system of conductors offers every possible guarantee of human safety.

We have referred to the Ferranti mains more particularly, perhaps, in connection with high-tension distribution, but it is apparent that they are also adapted for 100-volt or other low-tension distribution, or where a large mass of copper is required. By their construction the greatest amount of copper conductor is contained in the least space, the cables are buried direct without the use of conduits, and can be carried anywhere and under pavements where there is no room for conduits. The house connections are rapidly and cheaply made, perfect insulation is obtained, there is no fear of any short-circuiting from water, and no fear of explosions. For three-wire distribution the cable is manufactured with three concentric conductors instead of two, and is laid and jointed exactly as above described. We understand that arrangements are now made by Messrs. S. Z. de Ferranti and Co. with a large firm in the North of England for the manufacture of the Ferranti mains on a considerable scale, and large plant is being put down for this purpose.

PULLS AND BELL-REPEATERS.

For over 40 years the familiar push-button used in conjunction with electric bells has remained unaltered except as regards the artistic design of its exterior, notwithstanding that considerable inconveniences are connected with the use of this simple household fitment. The ordinary pushbutton must be pressed in a direction at right angles to the surface of the wall or other support to which it is fixed. Again, the contact made is not always of the best;

it is essentially a butt" contact, and possesses little or no self-cleaning powers. When definitely established, say, in the immediate neighbourhood of a bed, or of an office, or dinner-table, it is difficult to actuate these push-pieces from any other position. Pull-pieces have been devised, but these need to be pulled, as a rule, in one particular direction; flexible cord connections are also used, connected to "pear" pushes, but these again are seriously liable to derangement, and may be said to contain the elements of their own destruction. Generally speaking, there are K

C

other than in that of the axis of the rod, RR, will deflect that rod, and cause the shank of the knob to touch the ring, CC, and make the necessary contact. This contact needs no silvering or platinising, as the knob can at any time be given a circular motion, which will clean the contact surfaces and ensure the establishment of the circuit. If it be desired to actuate this contact-piece from a distance, it is only necessary to tie a fine cord round the horizontal groove on knob, K, and this cord can be led off in any required direction such as to a bed, a chair, or a dinnertable.

This bell-pull, which is also shown in Fig. 2, is made in various patterns and sizes. It can also be conveniently combined with a sound repeater. When a bell-pull or push is actuated, it is of immense convenience to be able

FIG. 1.

R

many instances of daily occurrence in which the shifting of a bed, of an invalid's chair, or of a writing-table necessitates the advent of the electric bell-hanger and the moving of the contact-piece from one position in the room to another. In order to obviate these inconveniences, Messrs. Siemens Bros. and Co. are now supplying, under license from Major Bagnold, a simple and effective arrangement of bell-pull which, no matter where fixed in a room,

RIMBAULS
FIG. 2.

to know that the electric bell has rung. Figs. 3 and 4 show a new form of sound-repeater, which, combined with the contact previously described, makes a most convenient and efficient fitting for this purpose. In Fig. 3 the repeater is shown complete with the nickelled steel bell dome, and in Fig. 4 without the dome. When the latter is screwed on, one pole of the electromagnet is presented to the sound-boss of the bell; as soon as the contact is closed and the circuit is intermittently interrupted at the distant "chattering" bell the bell dome of the repeater is set into vibration, and gives out a clear ringing sound sufficient to indicate that the distant bell has acted, but not so loud to be inconvenient to the occupants of the room in which the contact is made.

can be easily actuated from any point in that room by attaching a thin cord, and leading this cord away in the desired direction; a very slight pull on the cord is necessary to make contact and ring the bell. A diagram of the bell-pull and connections is shown in Fig. 1. An elastic rod of steel, RR, is set vertically with its lower end firmly fixed into a brass block, B; on its upper end is screwed a brass knob, K, the shank of which passes through a brass ring, CC; the conducting wires are attached to B and C. A slight pressure applied to K in almost any direction

FIG. 4.

The above-described system of ringing a steel bell magnetically can be applied in other ways. Thus, supposing it is desired to actuate several bells in series on one circuit, one of these can be an ordinary "chattering" bell and the others can be simply "sound-repeaters" without contacts. No difficulty of adjustment is experienced as in the case when ordinary chattering-bells are joined in series.

The special bell-pull and the repeater-bell are fully protected by patent, and Messrs. Siemens Bros. have been appointed sole manufacturers of the apparatus,

SHIPLIGHTING PLANT.

The accompanying illustration represents one of Messrs. Laurence, Scott, and Co.'s shiplighting plants, SL5 type. The output of the one illustrated is 65 volts, 140 amperes, at 260 revolutions per minute. The engine is well and substantially built, with very large bearing surfaces and ample lubricating arrangements, but otherwise calls for no special mention.

The weight of the dynamo field magnets is taken through brass packing-pieces on two strong ribs that run across from one side to the other of the bed-plate, whilst they are held in position by means of the muntz metal studs coming through the strong brackets on the side of

mutator of the above machine there are 110 sections, one turn of conductor to each section; but there are only 55 slots cut in the armature, each 355in. wide. This would be an excessive width, but in winding, each slot is made into two by means of three longitudinal strips of charcoal iron, running the whole depth and length of the slot, but insulated from both the conductor and the armature core. These strips reduce the width of each slot to 14in., and thus make the outside surface of the armature sufficiently even, magnetically, to entirely do away with the heating of the pole-pieces and noise that would have been caused by the wider slots. The armature of the machine is 13in. diameter outside, 11 in. at the bottom of the slot, and 9in. long. It is wound with four parallels of No. 13 wire braided. The magnets call for no special mention. They

armature core.

A very

the bed-plate, circular brass distance-pieces being arranged between these brackets and the magnet poles. rigid job is thus secured. The armature of the dynamo is of Laurence, Scott, and Co.'s usual type, the conductor being wound in slots milled into the plates which form the These plates have hexagon holes, into which the hexagon steel shaft is driven with insulatingpieces, so that the mechanical arrangement is exceedingly good, and the armature will stand an amount of rough usage and hard work that would be fatal to any other kind. It is absolutely necessary, if an armature of this kind is to be successful, that the opening between the projections and the periphery of the armature-i.e., practically the width of the slot should not exceed a certain amount. In practice, it is found it should not be more than twice the clearance between the armature and the pole-pieces. In the com

are of Lowmoor iron, and have a sectional area of 85 square inches. The ampere-turns in the shunt are 9,370, and in the series 7,840.

The plant illustrated was worked up to 200 amperes for six hours before leaving the maker's works. Even with this very excessive load there was no sparking, and the rise of temperature in the armature at the end of the test was only 80deg. F. The makers have recently obtained a license from Mr. Gisbert Kapp to use his patent end connections for strip armatures, and these will be used in all the large machines. They have been making a large number of shiplighters for Scotland and the North of England during the last 18 months. They have had an absolutely clean record with them, not a single breakdown having occurred up to date, and the buyers in every case have been thoroughly satisfied.

Frankfort in September last: "In a commercial country like England an invention to attract attention and to prosper must be patented. It must have the prospect of great gains to attract capital. It must therefore be developed by private enterprise. When a new industry has thus been developed, and when it affects the public weal, Government is forced to I have never seen the step in and take the control of it." license which the Government granted; it is, however, common knowledge that the Government reserve to itself the right to purchase the telephone companies at certain periods, one of which expired on June 30 last. Ought not the Government to have exercised this right in June last? I judge from what Mr. Preece said at Frankfort that the Post Office, wisely realising the importance of acquiring the telephones, and knowing they would never again buy them up so cheaply, were

Since my return to London I have read the correspondence that took place a few weeks ago in the Times, and the comments of the electrical journals on the proposed new departure as to telephones.

I am not a shareholder in and have no direct or indirect financial interest in any telephone company, but I have used the telephone for several years, and, in common with all who use it, I am interested in the discussion which took place in your columns during the vacation. The service in London is not perfect, yet it is most useful, and has been much improved since the amalgamation of the companies, and doubtless every improvement that can be made, under existing conditions, will be effected by the able men who direct the National Telephone Company.

Mr. Bennett's wish is to establish an ideal telephone service at very low rates of subscription, and naturally we all wish to have such a service. Many practical men say, and the electrical papers seem to agree, that Mr. Bennett's plan, so far as he has made it known, cannot be made commercially successful. It is certainly strange that the National Telephone Company should have allowed Mr. Bennett to leave their employment, if they believed he could render their service many times more efficient than it now is, while reducing the cost to subscribers, and at the same time enabling the company to earn larger dividends than they are now doing.

I have no wish to enter into the merits of the existing system as compared with that suggested by Mr. Bennett and others; for it seems to me that a very important public question is involved in this matter of telephones-one which concerns the good faith of the Government.

Shortly after the telephone company started, the Post Office brought an action against that company, claiming a monopoly of telephonic, as well as of telegraphic, communication. It is well known that the decision of the court of first instance was in favour of the Government; but it was felt at the time, and is felt by lawyers and others still, that if the company had appealed the decision of the court of first instance would almost certainly have been upset. The Post Office, however, offered the telephone company a license for 30 years, on their paying a royalty of 10 per cent.; the company not having much money, and shrinking from a costly legal fight against the Government, accepted those terms; and on the strength of this long license, far more than on their patents, a capital of more than two and a half millions of pounds has actually been paid by the shareholders and expended in the development of the system from the crude state in which it was when they commenced business. It seems now that the National Telephone Company is threatened with the active opposition of other companies licensed by the Post Office, and even by the Post Office itself. There is to be free trade in telephones in England, in face of the partnership in profits which exists between the Post Office and the National Telephone Company.

last, but, as the purchase was not made, I suppose this wise policy was overruled by the Treasury. I presume that, the period having elapsed, the Government have lost their absolute right to purchase the telephones in the present year. Surely, however, it will be better that they should approach the telephone company, and arrange to purchase all their interests under the conditions of the license, rather than that we should have an unseemly quarrel, and a number of rival companies starting, very few, if any of them, ever paying a dividend. Moreover, is it right that a Government department, having granted a license under which they benefit largely, and in consequence of which two and a half million sovereigns have been raised, should throw obstacles in the way of or even trade against their licenses?

We all want a perfect telephonic system. The Government could establish such a system better than anyone else, and, best of all, the telephone department of the Post Office would pay well, for the National Telephone Company is paying well. The rental for the year ending April 30, 1890, was given in the Electrician as £364,704. 17s. 5d., and for the year ending April 30, 1891, at £422,378. 6s. 2d. ; the balance to the credit of net revenue this year is given as £194,821. 8s. 1d.

The Government could do, at any rate, as well, perhaps better, as they can raise the necessary stock at a cheaper rate than any company. If the Government do not buy now, it will cost them very much more to acquire all the telephone companies at some future date, and this will be a great mistake. I do not know whether the telephone company wishes to sell; I feel sure that it will be for the public advantage that the Government should purchase.

THE ELECTRO-HARMONIC SOCIETY.

A smoking concert will be held on Friday, November 6, at the St. James's Hall Restaurant (Banquet-room), Regent-street, W., at eight o'clock. Artistes: Masters A. Dearden, F. Cooper, S. Newton, A. Wells, and F. Dalton; Mr. H. W. Nicholl and Mr. Alfred Moore; humorous songs, Mr. Herbert Schartau; solo flute, Mr. John Radcliff; pianoforte solo, Mr. Alfred E. Izard; musical directors, Mr. T. E. Gatehouse and Mr. Alfred E. Izard. A Broadwood piano will be used. The following is the programme :

"Go pretty rose.'

....L. Luzzi.

.Chaminade.

Mozart. .Marzials.

Masters A. Dearden, F. Cooper, S. Newton, A. Wells, and

Flute solo...........

F. Dalton.

"Scotch Fantasia."

Mr. John Radcliff.

Humorous song..

Mr. H. Schartau.

Song....

Flute solo

Song......

PART II.

"The Moon has Raised"......Sir J. Benedict.

("Lily of Killarney.")

Messrs. W. Nicholl and A. Moore.

Part Song............

"Twelve by the Clock "........................E. H. Lloyd.

Masters A. Dearden, F. Cooper, S. Newton, A. Wells, and