288 PROCEEDINGS OF THE AMERICAN ACADEMY. 



Two loaded coil d'Arsonval galvanometers have been constructed for 

 me by Mr. Thompson. The first (V), shown in Figure 1, Plate 1, is 

 about 76 centimeters high over all, and the gimp by which the coil is 

 hung is 32 centimeters long. The brass disk, which is 11.4 centimeters 

 in diameter, is rigidly attached to the rectangular frame (3 centimeters 

 X 7 centimeters) upon which the copper wire coil is wound, and is 

 accurately perpendicular to the axis of suspension. 



After the copper frame constructed for this instrument had proved 

 unsatisfactory, a cast type-metal frame was made to take its place. 

 Of course this frame is not nearly so effective in damping the swings 

 of the coil as a copper frame would be, but, on the other hand, its mag- 

 netic moment when it lies between the poles of the magnet of the gal- 

 vanometer is not large. The insulated copper wire on the frame, 

 however, gives a comparatively high moment to the whole suspended 

 system, and the period of the galvanometer is much shorter — only 

 about 140 seconds — than we supposed it would be with so large and 

 heavy a disk. The binding posts and all the other connections are of 

 copper, and the current is led into and out of the coil by two copper 

 spirals under the disk, so fine that they do not exert any appreciable 

 righting moment when the coil is deflected. The gimp is of steel, just 

 stout enough to hold up securely the loaded coil. 



The second galvanometer (W), represented in Figure 2, Plate 1, is 

 about 111 centimeters high over all and 30 centimeters in diameter ; 

 the suspension gimp is about 80 centimeters long. It seemed nearly 

 hopeless to attempt to get a sufficiently small righting moment with a 

 hollow coil made of such wire as was to be obtained in the open market, 

 so a coil of the Ayrton and Mather form was made for this instrument. 

 The disk is accurately mounted on a metallic rod upon which the coil 

 is fastened. The disk is built up of a thin sheet of flat aluminium with 

 a brass rim about 24 centimeters in outside diameter and 15 millimeters 

 in width. The current enters and leaves the coil through very fine 

 copper spirals, one above and one below. If No. 44 or No. 46 copper 

 wire be rolled out flat between jewellers' rolls or other similar device 

 the resulting gimp serves to make a spiral which has extremely little 

 torsional rigidity. It is possible to increase the number of field magnets 

 in this instrument at pleasure. The logarithmic decrement of the gal- 

 vanometer is small, but it has proved to be possible to bring the coil to 

 rest at its zero point without much difficulty. The complete time of 

 swing of the coil is about ten minutes, and the throws due to succes- 

 sive impulses of the same intensity agree with each other very closely 

 indeed. I am much indebted to Mr. Thompson for the great skill and 

 patience he has used in making these instruments. The apparatus was 



