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up as to what has been done since. I have an illustration here of a galvanometer made in that way [exhibiting a tube shown in Fig. 1]. You see that the mirror is as shaded as any one desires. The mirror lies right behind this hole.

I do not see that the author claims to have any particularly new modification of the D'Arsonval galvanometer. What he is endeavoring to do is to obtain a very sensitive galvanometer and

FIG. 1.

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a very portable one, and he does it by using spiral springs. cannot begin to get as sensitive a galvanometer by the use of a spiral spring as with a suspension. This galvanometer will not begin to give such good results as the galvanometer which the author has first described. A galvanometer suspended in pivots has a friction error that cannot be gotten rid of perfectly, and that would prevent it from being a very sensitive instrument, and

in the second place the torsion of those spiral springs is very much greater than a suspension of the orthodox character. His coil is wound on a copper form. This copper form was patented by Mr. Weston in 1888 and is owned, I believe, by the Weston Electrical Instrument Co.-was originally used by Mr. Weston, I believe, and I have no doubt could be used for experimental purposes, but certainly is not new with Lieut. Parkhurst. In fact, this identical galvanometer, very similar to its present shape with the exception of the shunt coils not being inside, was made in 1884 by Carpentier, and is illustrated in an issue of La Lumiere Electrique for that year. I do not suppose Lieut. Parkhurst has ever seen it. Examples of independent invention are very common. I might say about this portable form that there is another defect about it, if he is endeavoring to obtain high sensibility, and that is his magnets are separated by spaces equal to the thickness of the magnets themselves. Of course, he gets half the strength of the magnetic field that he would get if the magnets were solid.

I have a print of a galvanometer here-the galvanometer with which this tube goes and I will just pass it around for inspection. (All these details can be studied in Fig. 1.) You will see the magnets are laminated, but they are laid right on top of one another. These magnets are punched from the best English steel, punched with a die. They are laid one on top of the other. We get the maximum strength of field. The shape of the coil also in that galvanometer has been shown by Ayrton and Mather and several others', to be a shape that is not adapted to the highest sensibility. The coil as used in that illustration is rectangular in form. Now this wire lying horizontally, top and bottom, is dead wire, of absolutely no use in producing deflection. The moment of inertia of that coil is greatly increased by making it in that shape. It has been completely shown mathematically (in the article just quoted) and practically, that the proper shape of the coil is more like this-a very narrow coil the cross-section of which should be two circles and I verified that practically.

Now with reference to making a very sensitive D'Arsonval galvanometer portable, the usual difficulty of course has been-and in the instrument first described that difficulty exists-that with the coil hanging by a suspension it is utterly impossible to hold the coil; it will break away and break the suspension. The galvanometer illustrated by the blue-prin: (shown in Fig. 1) is a perfectly portable instrument and yet it has maximum sensibility. A tube contains the suspension and the coil. The coil hangs in the lower portion of the tube, and there is a suspension from the top of the tube to the coil, and a very light elongated spiral spring at the bottom. Inside of the tube, above and below the coil are a couple of flanges (seen in the tube at the bottom of Fig. 1.)

1. London Electrician, April 11th, 1890.

At the bottom is another tube inside this outer tube held there by friction; the lower flange is on the inside tube. When you want to transport the instrument or change its position all that is necessary to do is simply to slide this inside tube up and the coil is raised, the tension is taken off the upper suspension and the coil is securely clamped for transportation. I might say that this idea of clamping the coil is not strictly original with me. It was done by Professor Ayrton in a slightly different manner. In Professor Ayrton's instrument there was a hole in the side of the tube and a plug went into the hole; the coil was not raised, however, but was simply pushed to one side leaving the tension still upon the suspension which was thus liable to be broken.

As regards suspensions, we have used as fine as one mil wire rolled down into strips, the width of the strip being five or six times its thickness making an exceedingly delicate instrument, which makes it very easy on the 500-ohm instrument to get a constant of 1,000 megohms.

The question of damping with this instrument, is a question that can be regulated to any extent you like, to satisfy yourself. The damping is produced by means of this tube, which is of aluminium, very light, in which the coil itself is contained. The whole tube is rigidly fixed to the coil, and any desired damping can be produced by merely cutting a slot in the side of the tube to prevent the induced currents; this one has been slotted as you see and the desired damping has been produced.

There is one very important point in connection with galvanometers of this type which I have not seen touched upon anywhere. The author says that the constant of such a galvanometer once set up is invariable. Of course everybody knows that that meansif the magnet does not change. The magnet will change. But that is not the point I was going to bring out. The tension of the suspension is a very important factor in determining the sensibility of the D'Arsonval galvanometer. You can vary the sensibility by 40% or 50% by merely changing the tension with which that coil is suspended in the magnetic field, and consequently the constant is liable to daily changes produced by the difference in the coefficient of expansion between the supports for the coil, and the suspension itself. There, by the way, is the form on which the coils are wound for ballistic purposes where no damping is desired. [Showing a boxwood cylinder grooved upon the sides to receive the wire.]

Just one further point with respect to the author's remarks about manufacturers not furnishing sufficient data with instru ments. That is a very important thing, and as a representative of a manufacturer I cannot allow that to go unnoticed. I have probably seen as many imported instruments, if not more, than other man in the United States. A large percentage of all the instruments imported into this country I have seen, and I cannot remember more than three or four cases in which a foreign

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instrument was accompanied by data. You usually have to write. and get it if you want it. Often when you do get it, it is not the correct data. I remember a tangent galvanometer imported from Germany when I was at college. We had the data given, but it was utterly impossible to verify it. The only conclusion we could form was, that the data were erroneous by 20 or 25 per cent. However, instruments of precision in this country, so far as I know, are accompanied by all the necessary data. I should hardly call the Western Union type of galvanometer an instrument of precision. It never was intended for an instrument of precision, but was merely intended for testing in telegraph work and work of a similar character, and nobody expects to get an instrument of precision from it. It would be impossible to construct an instrument of precision at that price. It requires special skill, and men to do that character of work, and they are expensive. But real instruments of precision are accompanied by all the necessary data. In the manufacture of tangent galvanometers, with which I have had to do, there are a certain number of turns on each coil. The form is very carefully measured in the first place by a steel tape which is very accurately divided and its errors known. The thickness of the tape is allowed for, and from the circumference is figured out the radius. Then the radius of each layer is figured in the same way, and from all the radii the mean radius is obtained. In the making of resistance coils, standard instruments of precision of all kinds, all the data are furnished, so far as I know. I have here a number of certificates that I will just pass around which illustrates what I am referring to. [Passing certificates around.] For instance here is a certificate that accompanies a high-grade resistance-box. This certified that the box marked as stated has been adjusted with a certain percentage error at a certain temperature centigrade. Wire used, so and so, having a coefficient so and so, between so and so, and so and so degrees centigrade. That a greater potential than so and so must not be used. It says the connecter running from the bridge to the rheostat plus the plug contacts across the rheostat measures so and so. A good deal more is given than is necessary. I think it is only justice to American mannfacturers in general to say this. As far as I know they are all striving in every possible manner to make instruments in the best possible way and to furnish all the data that are necessary in connection with instruments and I think it is pretty thoroughly done.

MR. EDWARD WESTON:-The attention that has been called within the last few years to this type of galvanometer, commonly known as the D'Arson val galvanometer, has been quite marked. I think it has been the subject of six or eight papers during the last few years. It is somewhat remarkable that this type of galvanometer received very little attention from its first description by Maxwell in the first edition of his work, until comparatively within the last few years. It seems to me that Maxwell described

it very fully and in considerable detail, and Deprez and D'Arsonval took up the matter and seem to have been the first ones to construct an instrument embodying the moving coil and adapted for use as a galvanometer. That was some time about the year 1881. I believe; at least that is the date of the French patent. Later on they made some attempts to make a more generally useful instrument out of the galvanometer, and I think that is clearly shown in two French patents which were granted to them, one in 1881 and one in 1884. In the 1881 instrument of the patent, they had the form of field described by Lieutenant Parkhurst, namely, a series of permanent magnets fastened together in a vertical position with a moving coil between them, just as shown here, but with a soft iron tube in the centre instead of a solid core. Such a type of instrument is of very little value, excepting for use with a mirror and scale, or as a simple indicator of the difference of potential. As an absolute measuring instrument it is of very little value indeed, unless it is calibrated throughout a very large length of movement of the coil. Of course if the coil moves a certain distance, it changes its position in the field and the strength of the field naturally varies materially, so that such an instrument as that would have to be calibrated for a great many different positions. Later on D'Arsonval took out another French patent in which he placed pole-pieces, making a practically uniform field. That was, I think, in December, 1884; still the tubular form of core, and now the coil, in the same position in relation to the field. of course is a very great improvement. I think Mr. D'Arson val was quite some time behind me on that work. I have never been in the habit of saying very much about my work; I go on with it quietly; but I can say that long before the date of the patent, I had such an instrument in use and knew of its advantages and disadvantages. Later on the form of instrument which is commonly known as the Weston voltmeter and Weston ammeter was brought out by myself. That was put on the market in 1888. Of course it was not put on the market immediately when it was made. In that type, this same kind of field is employed, and I believe that that form of field, and that kind of arrangement was original with myself. I am not quite sure about

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it. I think so. But instead of the suspension, a pair of spiral

springs were used to carry the current into and out of the coil, and to supply the devices which measure the forms of the current passing through the coil. After the attention of the world at large seemed to have been called to the utility of this moving coil, more especially in the little portable instruments that were used and distributed throughout the world, a great deal of attention was bestowed upon this moving coil type of instrument, and Mather and half a dozen others, and probably still more, have taken up the matter and discussed it from various standpoints, and we have had such an instrument as Mr. Willyoung

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