Transactions of the American Institute of Electrical Engineers

ing of the American Institute of Electrical Engineers, New York, May 17, 1893, President Houston in the Chair.

A MODIFIED DEPREZ-D'ARSONVAL GALVANOM

ETER.

BY LIEUT. CHARLES D. PARKHURST.

So far as the writer has observed in all forms of this galvanometer in actual use, or as illustrated and described in various articles descriptive of its construction and use, and in catalogues advertising it for sale, it has always been arranged for use with a lamp stand and scale for reading the deflections by the varying position of the spot of light and image of cross-hair reflected from a concave mirror; the protective screen to cut off air currents has always been of glass, and even though a plain mirror be substituted for the concave, there is nothing to put the mirror in shadow; hence, as the writer knows from experience, no satisfactory results could be obtained with this form used in connection with a reading telescope.

Opinions may differ as to the relative merits of the two systems of reading reflecting galvanometer deflections. Certain it is, however, that the lamp stand and scale, form a clumsy and bulky outfit. To get a good, sharp and clear image of the spot of light and its cross-hair needs a very strong light or a dark closet in which to use the galvanometer. Independent of its bulkiness and want of portability, it is a hot and inconvenient method of reading, and one that, in the writer's opinion, bears no comparison with the ease and refinements of reading, obtainable with a reading telescope worthy of the name.

Since to show up well in a reading telescope the mirror must be in a heavy shadow, and the scale upon the telescope be well illuminated, the writer found that a modification of the ordinary D'Arsonval galvanometer in his possession was necessary, and

this modification will now be described, reference being made to the drawings herewith drawn to scale one-quarter size.

The horse-shoe or rather U-shaped magnet was dropped down completely through the base, so as to project below its lower surface sufficiently to allow the lower tension spring to be mounted on the under side of the base, where it could be reached at any time to adjust the tension of the suspension. This became necessary owing to the fixed brass case which enclosed the instrument, as shown in the illustration, this fixed brass case being the necessary opaque case to shut off all light except from the front, and at the same time protect the interior of the instrument from all air currents. The leveling screws of the base were made long enough to allow the tension spring screws to be conveniently

reached without disturbing the galvanometer when once set up and leveled. All adjustments necessary at any time could therefore be made without any undue disturbance of the galvanometer on its stand. This tension spring is shown at s, Figs. 2 and 3; the check nut and tension screw at c and T, same figures.

Although the construction of the ordinary D'Arsonval galvanometer is probably known to all, the following complete description is given of the modified construction for the benefit of any who may not be familiar with the instrument in any form, and also to enable the modification to be fully understood and be copied by any one desirous of so doing. Fig. 1 shows the plan of the base with the magnet in position; double brass knees are used to ensure the vertical and rigid position of the magnet.

(See K K, Figs. 1, 2 and 3.) In Figs. 2 and 3 it will be seen how the magnet is dropped down through the base, giving room between the bottom of the base and the bend of the magnet for the point of the tension spring to extend into the centre line, so as to receive the lower suspension wire. flat strip of spring sheet brass about at the securing end, and inch wide mounted upon a block of brass

The spring itself is a inch thick, inch wide at the free end. It is at the securing end, and

here rigidly secured to the base; this gives plenty of free play

for the spring at its free end and it carries a small silver hook securely soldered into the spring.

The base itself is turned up from solid ebonite, with a raised central part that fits the bottom of the brass case snugly, thereby centering the latter upon the base; the brass case is of 4 inch brass tubing, inch thick, provided with the window as shown. The window is made large to allow the mirror and its reflected scale to be seen through a wide angle.

The frame of the instrument to provide suspensions for the

coil is entirely independent of the case. It consists of the upright brass rod u and the upper bar T. The upright is securely fastened to the base in rear of the centre of the magnet as shown, and at right angles to the base.

The bar T projects over the top of the magnet, as shown, and carries upon it, as an integral part, the circular flanged plate P. This plate fits into a corresponding central hole in the top of the brass case, so as to close this latter hole snugly; this hole is necessary so that the case can be removed at any time without disturbing the suspension, or can be put on after any adjustment or

D

M

R

FIG. 3.

repairs that require the removal of the case, and yet leave the suspension untouched.

In the centre of the plate P, and so as to be exactly over the centre of the magnet, the tube R is securely fastened; this tube carries and centres the sliding rod A; this latter rod carries at its lower end the silver hook as shown, and is clamped in any position desired, by the screw w and check nut N.

The coil frame F (for details see Figs. 4, 5 and 6) is made from thin sheet copper, worked over a form into a light rectangular