126 PROCEEDINGS OP THE AMERICAN ACADEMY 



There were twelve coils, six on each side of the movable coil which is 

 shown with its suspension between them. The large coils were insu- 

 lated from each other by vulcanite washers, and held together by brass 

 rivets which were insulated by vulcanite cylinders. The coils were 

 placed at a distance apart equal to their thickness, and thus allowed 

 currents of air to pass freely between them. This arrangement is 

 shown at I, Fig. 2. The bifilar suspension is connected with a gradu- 

 ated circle which read by means of verniers to one minute. The tan- 

 gent and clamping screws of the torsion head are not shown in the 

 figure. The general arrangement was similar to that used by Mr. 

 Latimer Clark, and figured in Maxwell's " Electricity," with the 

 exception that the graduation was not upon a cylinder, but was on a 

 plane, and the graduated circle was such as is used on spectrometers. 

 The torsion head admitted of vertical adjustment by means of the hol- 

 low cylinders at its points of sivpport, in addition to the vertical adjust- 

 ment of the pulley by means of which the tension upon the suspending 

 threads was equalized. In the ordinary form of electrodynamometer, 

 the current passes down one suspending wire and up the other. In 

 my dynamometer, this is not the case, as is shown in Fig. 2. There- 

 fore, the suspension can be made of strands of silk or any suitable 

 material, according to the sensitiveness desired. In the actual use 

 of the instrument with powerful current, it was found necessary to 

 use steel wire in order to increase the directive force, so great were 

 the deflections. 



The movable parts are best shown in Fig. 2. The construction of 

 the central coil is shown at D. The water enters at a, passes out at 

 a after cooling the hollow chamber B, which admits of adjustment, 

 and then flows by rubber tubing toy*; and, after cooling the mercury 

 cup E, flows out through f. G is the water-chamber which answers 

 to B. At n, below the mirror m, is a bar upon which are hung cylin- 

 drical weights to determine the moment of inertion to alter the sensi- 

 tiveness. Only one coil and a half are shown in the figure. The 

 electric current enters at H, passes through the mercury cup to E, then 

 to C, and thence by the hollow cup to O, and then around the outer 

 coils. 



A telescope with scale was employed to read the deflections ; but it 

 was found better, in practice, to use the graduated circle of the torsion 

 head and bring the movable coil back to zero. In this case, we have 

 from the theory of the electrodynamometer : — 



c ' 2 = yj^ sin d; 



