Electric Currents of great Strength. 167 



rents of air to pass freely between them. This arrangement 

 is shown at I (fig. 2). The bifilar suspension is connected 

 with a graduated circle which reads by means of verniers to 

 one minute. The tangent 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 gradua- 

 tion was not upon a cylinder, but was on a plane, and the gra- 

 duated circle was such as is used on spectrometers. The 

 torsion-head admitted of vertical adjustment by means of the 

 hollow cylinders at its point of support, in addition to the ver- 

 tical adjustment 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. Therefore the suspen- 

 sion 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 construc- 

 tion of the central coil is shown at D. The water enters at a, 

 passes out at a after cooling the hollow chamber B, which ad- 

 mits of adjustment, and then flows by rubber tubing to/, and, 

 after cooling the mercury-cup E, flows out through/. Gr is 

 the water-chamber which answers to B. At n, below the 

 mirror m, is a bar upon which are hung cylindrical weights 

 to determine the moment of inertia to alter the sensitiveness. 

 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 0, 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 electrody- 

 namometer, 



and the effect of the earth and local attraction are eliminated. 

 By this method of observation no telescope and scale are 

 needed : it is only necessary to bring the point of the bar 

 which passes through the movable coil to a fixed point. The 

 mercury in the pivot-cups serves to damp the vibrations of 



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