Vol. XVII.

DISCHARGE OF FIELD MAGNETS WHEN CONNECTED TO BUS BARS.

BY

Wine A. Anthony.

FIND in Mr. Herrick's paper on "Central Station Switch Boards," which was read at the Washington meeting of the National Electric Light Association, the following passage referring to the method of handling dynamos where the fields are excited from the bus bars:

The disadvantage of this method is, that the field is on, although the dynamo may be shut down, and it is necessary to discharge it through a bank of lamps or other resistance by means of a field break switch. This method of discharge, although safe, might cause trouble should the bank of lamps be disconnected or otherwise out of order.

This passage calls to mind discussions that I have many times had with electricians as to the necessity of discharging the field through a resistance. Why not short circuit it? The impression seems to be quite general that a resistance approximately equal to that of the field should be provided, and the switch which breaks the field circuit. arranged to discharge the field through this resistance. I have never been able to obtain a clear statement of the reason for this belief. The Mather Electric Company has for several years constructed the field rheostats for large machines, and especially for 500 volt generators, to admit of the connections shown in the sketch. As the rheostat lever is turned toward the right, resistance is introduced

No. 310.

into the field circuit, gradually reducing the current until, when the lever passes off the point a, the circuit is broken. But at the instant of passing off the point A, contact is made with the point B, which short circuits the field and permits of its discharge through a circuit C D E F, of practically no resistance.

I have been informed that the electrical engineer in charge of the erection of one of the Mather plants insisted that their agent who was having the plant installed, should have a resistance inserted in the branch B E. The paragraph quoted at the beginning of this letter recalled this matter to my attention and leads me to state the theory of the action as I understand it.

When the field circuit is broken, the dying out of the magnetism develops an E. M. F. in the field windings tending to produce a current in the same direction as that which magnetized the field. If no path is offered through which this current can flow, this E. M. F. develops a difference of potential between the field terminals and between different parts of the field winding, that may amount to hundreds or thousands of volts, sufficient to break down the insulation and cause serious damage to the field coils. If, however, a path of no resistance, as C D E F, is provided between the field terminals, the E. M. F. will be almost wholly employed in producing current through that circuit, and no appreciable potential difference will be developed The current through the short circuited field can never be greater than that which originally excited the field, because it is produced by the falling magnetism, is in the same direction as the exciting current, and, if it should equal it, the magnetism could not fall. Further, the potential difference between F and c can only be such as would maintain his current through the resistance C D E F, and the smaller this resistance, the less this potential difference. Further still, the less the resistance in C D E F, the greater the current developed by the falling magnetism; the greater the current the more slowly does the magnetism fall, and the more slowly the magnetism falls the less is the E. M. F. There is every reason then for making the resistance of C D E F as smail as possible.

This arrangement of the rheostat saves the necessity of an extra field switch, and insures the partial cutting down of the exciting current before it is finally broken.

GROUNDS ON INCANDescent CIRCUITS.

BY

Geo. L. Thayer.

LOCATING grounds on electric lighting plants or installations is one of the duties which the average station manager always regards as extra work, principally because they seem to come at the most inopportune moments. Then it is so easy to let it go" until a more convenient time comes to hunt it up.

Grounds are naturally divided into two classes, outside and inside grounds. Where the line is in a conduit, the location of trouble requires special methods, depending upon the construction. On pole work grounds can usually be easier located by actual inspection of the line than by testing. The great majority of grounds are inside, and are

caused by contact with the iron pipes in the building. They are concealed, and electrical methods must be used for their detection and location. The larger wires, carrying heavy currents, are generally run in places where they are not apt to be disturbed and their extra thickness of insulation is more or less a protection against mechanical injury. The result is, that most of the grounds occur in fixtures or in drop cord sockets resting against gas fixtures or steam pipes.

In practice it is usually better not to test large sections of the installation at one time when the current is on, because of the burning of connections when circuits carrying, say, ten amperes or more are opened. The wires too are heavy and frequent bending is liable to break them.

Testing with a magneto, or with a galvanometer and battery generally necessitates a search for something on which to ground one side of the testing apparatus and running a wire to it, all of which takes time. On alternating current circuits this is of course the best method, as there is no electrical connection between the primary and secondary circuits. But in direct current plants the ground detector may be used as the necessary ground. When current is on, the only apparatus needed is a socket wired with a few feet of wire or cord and a 16 c. p. lamp. When current is off, a magneto must be used instead.

Referring to the accompanying diagram, let the cross denote a ground on the negative side of the system. When the ground detector switch is closed, current flows from the dynamo through the ground detector lamp to ground, entering the system through a ground at some unknown

GR.

LOCATING A GROUND.

point. Now if a lamp be placed anywhere in this circuit, say at the cutout, it will light up and indicate the faulty

circuit.

If the circuit is broken at the cutout and the lamp cut in, the lighting up of the lamp to low incandescence will locate the ground on that particular branch. Taking one cutout at a time, both sides are opened and the lamp placed in the negative side of the cutout, assuming of course that the ground has been shown to exist on the negative side of the circuit. The lamp will show at once whether that branch is clear or not. If no lamps are burning on that circuit, the positive wire need not be opened. Each circuit is tested in a like manner until the trouble is located in some one of them when it can be further located by the usual methods. Of course the absence of results shows that it is somewhere on the mains and feeders.

Testing may be begun by testing the main cutouts. The arcs formed when the heavier currents are broken burn the binding posts. The wires are No. 12 and larger and frequent bending is apt to break them off; besides, the trouble will usually be found in the end on the smaller branch circuits. In central station work the building should of course be located first. Where plug cutouts of the Edison type are used, the testing lamp may be screwed directly into the cutout. In city plants there are always a few lamps burning and it is very desirable to interrupt.

the service as little as possible. One man can cover as much ground with a lamp as two can with a magneto. If the ground is of very high resistance, a cheap galvanometer should be used instead of the lamp. The time at each test is small, but for an entire plant, it amounts to a surprising difference.

It may be urged that grounding the system in this way in order to test it, is bad practice on account of fire from the heat generated at the point of trouble; this is true, but the danger is more apparent than real. A dead ground, or one of above 1,000 ohms resistance would not be apt to give any trouble by fire. How many plants run along with grounds on both sides of the system? It would take time to develop a fire from this cause, and the testing of the largest isolated plant need not exceed two hours. At the end of the test the men would have located the ground and an incipient fire would run an exceedingly small chance of getting a good start. In any case the chance of danger can be avoided by making the artificial ground of high resistance and using a galvanometer, which, after all, is the better way. Right in here comes the subject of blowing out grounds. Don't; at least, don't make a practice of it. It is at best a heroic remedy. In any event the general location of the ground should be ascertained first and a careful watch kept for fire for some hours afterwards.

THE ELECTRO-MAGNET; or JOSEPH HENRY'S PLACE IN THE HISTORY OF THE ELECTRO-MAGNETIC

TELEGRAPH.-IX.

BY

Mary A. Henry

Henry's paper was in print.' In the wide circulation of Silliman's Journal it was carried abroad to place Henry at once among the scientific men of Europe. "At one bound" says Prof. Lovering "he came to the front. He had introduced his magnetic children into the world; had sent them forth to do their important work and how were they received? With surprise, nay with astonishment."

Mr. William B. Taylor says, "The magnetic 'spool' of fine wire, of a length tens and even hundreds of times that ever before employed for this purpose, was in itself a gift to science which really forms an epoch in the history of electro-magnetism. It is not too much to say that almost every advancement which has been made in this fruitful branch of physics since the time of Sturgeon's happy improvement, from the earliest researches of Faraday downward, has been directly indebted to Henry's magnet. By means of Henry's 'spool' the magnet almost at a bound was developed from a feeble childhood to a vigorous manhood. And so rapidly and generally was the new form introduced abroad among experimenters, few of whom had ever seen the paper of Henry, that probably very few indeed have been aware to whom they were really indebted for this familiar and powerful instrumentality."

"But in addition to this large gift to science Henry (as we have seen) has the pre-eminent claim to popular gratitude of having first practically worked out the differing functions of two entirely different kinds of electro-magnet; the one surrounded with numerous coils of no great length, designated by him the quantity magnet, the other

surrounded with a continuous coil of very great length, designated by him the intensity magnet.

"Here for the first time is experimentally established the important principle that there must be a proportion between the aggregate internal resistance of the battery and the whole external resistance of the conjunctive wire or conducting circuit.

"Never should it be forgotten that he who first exalted the quantity magnet of Sturgeon, from a power of twenty pounds to a power of twenty hundred pounds, was the absolute creator of the intensity magnet, and that the principles involved in this creation, constitute the indispensable basis of every form of the electro-magnetic telegraph since invented."

Faraday, working away at an experiment that had failed him again and again, immediately adopted the new method of obtaining magnetic force, to make, in this same year, 1831, that discovery of magneto electricity I have noticed in the last chapter, and by its means also his subsequent discoveries of dia-magnetism and the magnetic effects on polarized light were made. 3

A very pleasant welcome to the magnets was the generous tribute to them of Sturgeon. He says, "Henry has been enabled to produce a magnetic force which totally eclipses any other in the whole annals of magnetism, and no parallel is to be found since the miraculous suspension of the celebrated oriental imposter in his iron coffin.4

At home Dr. Hare, of Philadelphia, received with enthusiasm the new means of producing magnetic force. He says in a letter to Sturgeon, the following year April 5, 1832: "As soon as I heard of the wonderful magnets of Prof. Henry, I repeated his magnetic experiments, and I have recently made a magnet by means of copper wire shell-lac varnish and paper, surrounding the iron, which in proportion to its weight holds more than his." Prof. Silliman evinced his appreciation of the magnets by ordering the powerful one which continued to be an object of veneration and pride at the University of New Haven until last year, when it was sent to the National Museum for exhibition.

than the great magnet in Philadelphia." July 21, he writes-"Your magnet performs admirably well and excites great interest among the students. A favorite mode of exhibiting the magnet is to allow a number of young men to stand upon the scale, as the weight lifted then appears more striking. This never fails to excite the astonishment of the spectators."

This magnet was nearly twelve inches high and was made to lift 2,035-2,063 lbs. A number of very interesting experiments were made with it before it was sent to New Haven. "To test its power of producing magnetism in soft iron, two pieces of iron of an inch in diameter and 12 inches long, were interposed between the extremities of the magnet and the armature, and these when the battery was immersed became so powerfully magnetic as to support 155 lbs. To exhibit the effects produced by instantaneously reversing the poles-the armature was loaded with 56 lbs. which added to its own weight made 89 lbs. One of the batteries was then dropped, when the weight of course continued to adhere to the magnet. The other battery was then suddenly immersed when the poles were changed so instantaneously that the weight did not fall. That the poles were actually reversed in the experiment was clearly shown by a change in the position of a large needle placed at a small distance from the side of one extremity of the horse-shoe." 5

The venerable president of Yale thus speaks of the magnet :

"There was nothing to be said when, as the plunger went down into its bath, the impotent bar of iron became possessed of a giant's strength and could pick up and hold a weight of more than a solid ton, and as the same plunger was lifted this gigantic energy vanished as at the word of an enchanter. The speaker well remembers the excitement which this discovery occasioned when the first experiment was tried at Yale College, in the presence of a few spectators who casually met at the call of Prof. Silliman, who was glowing with animation and delight. The ponderous platform was loaded with pig-iron and other heavy weights, with a few slight additions of living freight. Among the last was the speaker, being the lightest of all, and therefore convenient to serve on the sliding scale. It is more than fifty years ago but the scene is as vivid as the events of yesterday. The question went around who is Professor Henry, and how did it happen that nature revealed to him one of her choicest secrets. Thoughtful men asked what is this wonderful protean force which he was the first to follow in its sinuous hiding places and evoke by a magician's wand; and what are its relations to its kindred agents, and, above all, to the matter about us which we can measure and weigh and see and handle. To some it seemed but a successful guess by a daring adventurer. A lucky accident like the drawing of a prize in a lottery. It was not so with those who retraced the successive steps of close observation, of sagacious interpretation, of boundless invention, of ingenious construction, of patient trial, of loving sympathy which preceded this single achievment and all of which combined lifted at once this youth, hitherto unknown, into the rank of the most eminent discoverers, brilliant as was their company, then and since."

In his letter accompanying the account of the magnet in his journal Prof. Silliman says:—

"The magnet is now arranged in its frame, in the laboratory of Yale College. There has not been time, since the magnet came just as this number was finishing, to do any thing more than make a few trials which have, however, fully substantiated the statements of Prof. Henry. He has the honor of constructing by far the most powerful magnets that have ever been known. And his last, weighing, armature and all, only 824 lbs. sustains over a ton. It is eight times more powerful than any magnet hitherto known in Europe and between six and seven times more powerful

3. See Faraday's Researches.

4. Phil. Magazine, March, 1832.

5. Henry's paper in Silliman's Journal, for April, 1831.

A

Innumerable applications for magnets now poured in upon Henry, and for other magnetical apparatus. powerful magnet equal to the one sent to New Haven was ordered by Prof. Cleveland for Bowdoin College, Brunswick, Maine.

The making of the magnets consumed much time and labor but Henry in every case refused remuneration for his personal labor, charging only the actual cost of materials and hired work. "I consider it," he says, "below the dignity of science to take pay for my knowledge." (Letter Nov. 4, 1831.)

ore.

Not only these new galvanic magnets occupy Henry's attention: we find him engaged in preparing a large steel permanent magnet for use in the separation of iron from This inspires him with the desire to bring his own electro-magnets into the work. In a letter sending this to Mr. Rogers, Nov. 4, 1831, for whom it was constructed, he says, "I am confident I can form a galvanic machine which will effect powerfully the separation of iron, and upon this effort he spends much time and thought.

A detailed description by Professor Henry of his mode of constructing an electro-magnet may be interesting. I give part of a draft or copy of a letter to Professor Cleveland :—

ALBANY, May 8, 1832.

After a delay, which I fear has nearly exhausted your patience, I have at length sent off your magnet according to the direction given in your letter of the 8th of Dec. I can get nothing done in Albany in the philosophical line except I stand over the workman continually myself, or which is most often the case, do the work entirely myself. * *

*

*

The following is a particular description of the construction of the magnet.

*

*

* *

The horse-shoe is formed of a bar of American iron, which ac

6. Prof. Silliman is here alluding to permanent magnets.

cording to the mechanic's account was unusually hard. It was not selected on this account, but was taken because it was the only piece of iron of the proper size to be procured at that time in Albany. After bending it into the proper form, the edges were first rounded with the hammer and afterwards with a file, and in order to prevent the wires, to be coiled around it, from slipping off the legs, a deep groove was filed into each, about half an inch from the end.

The horse-shoe, when it came from the hands of the finisher, weighed 60 lbs.-the armature about 20 lbs., and these are almost precisely the weights of the magnet and armature of Yale College. The winding on of the wires was done with great care, and under my constant inspection, according to a method which I think much preferable to any I have before adopted. Instead of covering the wires with cotton or silk-thread, I gave them several coats of varnish made of shell-lac and mastic, and in order to render the insulation of the several wires still more perfect a thickness of silk was woven as it were, between every spire or turn of each wire, and the several layers of wires were separated from each other by a covering of silk and varnish.

The operation was as follows: The iron horse-shoe was, in the first place, covered with a coating of varnish, and while this was yet soft, the whole was wound with strips or ribbons of silk. A coating of varnish was then given to the silk and suffered to dry before the winding of the wire was commenced.

In coiling on the wire, one spire was passed around the horseshoe, with the end of a long and broad flap of silk between it and the iron the flap was then turned back so that the second spire should pass under the silk-the third spire passed over the silkand the fourth again under it-and so on in this way until the whole surface of the horse-shoe was covered with one thickness of wire. A coating of varnish was then given to this surface of wires, and the whole covered with ribbons of silk. Another coating of varnish was given to the silk, and after this became hard a second layer of wires was coiled on in the same manner as the

first.

This process was a very tedious one, and occupied myself and two other persons every evening for two weeks. It is, however, one which insures success if the iron and other circumstances are favorable. The iron is entirely covered with four thicknesses of wire, and near the ends with five. There are in all 30 strands, each 35 feet long, so that exclusive of the projecting ends there are about 1,000 feet of wire actually coiled around the magnet. It is necessary to be very cautious that in the arrangement of the several wires there are none which will conduct the galvanic current in an adverse direction, or that will suffer it to pass from one wire to the other without circulating throught the entire length around the magnet. I have failed in this respect in some instances to produce any effect when I expected a very great one.

****

To exhibit the experiment of the instantaneous change of polarity, a second battery must be attached by means of the thimbles of mercury, in such a manner that the galvanic current from this battery will circulate in an opposite direction to that from the battery attached permanently to the magnet. Load the armature with two or three hundred pounds and excite the magnet by the second battery; let an assistant be ready to raise the vessel from immersing the first battery. Let this be done suddenly, and at the same time quickly withdraw the wires or poles of the second battery from the two thimbles-the weight will still continue to adhere. To render the fact evident of the change of polarity, I place two magnetic needles one on each side of the magnet, and these, by reversing their position relative to the magnet will indicate in a very striking manner the change of polarity. I find it most convenient to make these needles each of two pieces of watch-spring, about ten inches long, touched separately, and then with their north poles joined together by a silk thread with a little brass cap between them. A small piece of light cord is placed on each end with the letter N on one and s on the other.

In the box containing the battery you will find two pieces of round iron; these are for showing induction of magnetism in soft iron. They must be placed upright on the face of the armature, at such a distance from each other that their axes will be in the centre of the faces of the horseshoe. While in this position im. merse the battery and the two iron cylinders will adhere to the magnet and the armature to them, as firmly almost as if they all were but one piece.

N. E. L. A. STANDARD RULES.

WE have received copies of the new N. E. L. A. Standard rules for lighting, wiring, &c., put up in a very neat pocket size, 32 pages, 9 by 534 in. They are supplemented by the report on arc lamp rating and by a glossary of electrical terms. There are also a preface and an index. These rules represent a great deal of work and are a great credit not only to the Association but to the successive committees, Mr. W. J. Hammer being the active and efficient chairman of the one reporting to the Washington convention. Work of this kind still remains to be done, and it is but to be hoped that the Association will continue to enjoy such devoted and intelligent assistance.

THE MERCADIER AND ANIZAN MICROPHONE
REDUCER.

Taking the condition of telephony as we find it to-day in different countries, and considering the progressive development of long distance lines, transmitters ought to be so arranged that one subscriber shall be able to talk as well with another in the same city as with one a hundred or two hundred miles off. Necessarily it is the microphone which ought to be studied in order to obtain as good talking for short as for long distance.

With microphones embodying carbon pencils it is necessary that the microphone contacts be very sensitive and very light when used for long distance work. One would then be quite limited by the noise caused by the too sensitive microphone in the telephone of the person talking; but this can be counteracted by means of a differential winding. To communicate over a short distance in a city district with copper wires it would be necessary, on the contrary, to have the microphonic contacts not very sensitive. In granulated carbon microphones one could vary at the same time the surface contact of the carbon electrodes and their distance, according to whether one desired to use the microphone for long or short distance.

Finally, whatever be the type of microphone, whether carbon rod or granules, one can also for a given regulation of the microphone contacts make the same microphone serve for long distances by taking a thin diaphragm of large diameter; and for short distances by mounting the microphone on a thick diaphragm of very small diameter. One can get an analogous result by taking diaphragms

of the same dimensions, but of different materials, such as iron and wood. In the United States the American Bell Telephone Company use two types of microphone transmitters, one for city and one for long distance work. In Europe there are still employed microphones regulated in such a way that they can serve either for short or long distance, but, as indicated above, these microphones do not give all that could be got out of them for long distance work, in order not to impair too much their clearness for short distances, and vice versa. It will thus be seen that the two desirable qualities in the regulation of the microphone, loudness and clearness, are opposed to each other, and that one can be obtained only at the expense of the other.

The microphone reducer of MM. Mercadier and Anizan has for its object, first, to give to a microphone its maximum intensity when used for long distances, and, second, to obtain from the same microphone a proper loudness and clearness when it is used for short distances. The diagram, Fig. 1, represents diagrammatically the arrangement which consists simply in placing for a given time a resistance in shunt to the carbons. This shunt s has about three ohms resistance. I is an ordinary switch for opening and closing the shunt circuit. The whole can be placed inside the transmitter boxes with a small handle projecting through the lid, which operates the switch 1. When the index on the handle points to " City" the switch is closed, and when pointing to "Long Distance" the switch is open,

Two terminals on the outside connect the reducer with the carbons of the microphone.

It has been possible to sensitize the microphone so as to make it as powerful as desired. If differential telephones are not used one would nevertheless be limited up to the point where the sputtering sound would become annoying to the talking subscriber. A single experiment on an artificial line having resistance and capacity allows the maker of the microphone to find the proper point of regulation. The diminution in loudness of the microphone so arranged is explained by the fact that, a part of the current passing through the shunt, the variations of resistance of the carbons only act on the part of the current which passes through the carbons. The clearness is good because whatever be the sensitiveness or mobility of the carbons the maximum resistance of the vibrating carbons in combination with the shunt can never exceed the resistance of the shunt. If we call C the resistance of the carbons and S that of the shunt, CS the total resistance will be R = C+S

Naturally, Cvaries

at each instant when talking before the diaphragm of the microphone. We may conclude from what precedes that there ought to be a benefit by placing a fixed shunt around the carbons of microphones which are too sensitive and subject to sputtering.

It is important to determine by experiment the value of the resistance of the shunt. It has been observed, in fact, that if a resistance of 10 ohms be placed in shunt around the carbons which have themselves a resistance of 10 ohms when at rest, the strength of the telephonic currents is scarcely diminished. If, on the one hand, half the current passes through the shunt we must not lose sight of the fact that, on the other hand, the combined resistance of the carbons and the shunt is only five ohms. The strength of the current which passes through the primary circuit of the induction coil is greater, and while the variations in resistance are only produced in the branches (carbons) of the derived circuit it happens that the variations of the current strength in the primary circuit of the induction coil are of the same order and magnitude as those produced when the shunt does not act, the switch being open.

There ought to be still another means of reducing the strength of current in the primary microphone circuit. This would consist in introducing in the circuit a given resistance R, Fig. 2. But one would thus obtain only one half the effects which the preceding arrangement gives. The loudness, it is true, would be diminished, but the clearness would suffer.

We may decide, therefore, either in favor of a shunt or of a combined shunt and resistance. Fig. 3 shows the latter arrangement diagrammatically. A three point switch replaces the switch of Fig. 1. When used for long distances the shunt circuit would be open and the resistance R would be short circuited. For local service the resistance R would be reduced in the microphone battery circuit, while the shunt circuit s would be closed around the carbons.

The employment of the shunt has given rise to very simple and inexpensive apparatus devised by MM. Mercadier and Anizan, of Paris, which can be readily combined with existing transmitters.

STEERING BY TELEPHONE IN A FOG.

THE London Pall Mall Gazette in a recent issue, describes the experiments of Mr. Charles A. Stevenson for locating the position of vessels at harbor entrances when, during certain states of the weather, other observations cannot be easily made. He proposes that a cable be laid in the sea, and the positions of vessels passing near or over it determined by means of a detector on board.

The first instrument used by Mr. Stevenson is a coil of uninsulated copper wire dipping into the water at the bow of the vessel and a similar water connection at the stern. These are joined by a wire with a telephone in the circuit, and a very sensitive instrument is produced. If the water connections are equidistant from the cable, as they would be if the boat were immediately over it, or lying broadside on, no sound is heard. The cable may be insulated or uninsulated and the action is similar with an induction coil.

With the coils separated ten feet (at the bow and stern of a small boat put down from the vessel) and an insulated wire 400 feet in length and laid through a small lake of brakish water fifteen feet deep, the alternations produced by a magneto-electric machine were perfectly distinct at the end of the lake 340 feet away from the wire, and the limit of audibility could not be ascertained.

Mr. Stevenson's second instrument is a coil of insulated wire surrounding a core and a telephone receiver in the circuit with the coil. With this instrument the making and breaking of the current produced through a wire 200 feet in length could be detected through 60 feet of salt water. The sound in a Bell telephone with the instrument was almost deafening with fifteen feet depth of water. This electromagnetic system of induction, in contradistinction to the parallel wire system, has no earth connection, being entirely insulated, and is therefore a case of true induction through water.