Velocity of the Electric Current. 105 



made contact, the current traversed the windings and made 

 exit through the opposite disc, axle and support. 



The coil thus mounted was driven by a belt. It could be 

 given a very high velocity of rotation and could be supplied 

 with current equally well whether at rest or in motion. In 

 some preliminary trials to determine the rate at which it could 

 be driven with safety the source of power was a small high- 

 speed water-motor. It w&s found that four hundred revolu- 

 tions per second could be readily obtained and that such a 

 velocity could be maintained without undue heating of the bear- 

 ings. Upon one occasion a speed of nearly six hundred revo- 

 lutions was reached, when the brass retaining band parted with 

 a loud report and the bobbin was instantly stripped of every 

 vestige of wire. The coil was then rewound and balanced 

 anew, and the rate of four hundred revolutions per second, al- 

 ready determined as lying well within the limits of safety, was 

 not exceeded during the remainder of the investigation. 



The rate of revolution was readily and accurately determined 

 from the pitch of the note emitted by the revolving coil, this 

 determination being verified from time to time by two inde- 

 pendent methods ; namely from the siren-like note uttered by 

 four screw-holes situated 90° apart upon one of the brass discs 

 attached to the bobbin, and by estimating the velocity of the 

 driving belt. 



The needle, by means of which variations in the magnetic 

 moment of the coil were to be detected, was placed immedi- 

 ately above the latter and as near to the windings as possible. 

 It consisted of a steel wire, about l om long and 1 millimeter in 

 diameter, hardened and magnetized. It was suspended within 

 a cylinder of copper and was rigidly connected with a precisely 

 similar needle, by means of an aluminium support, the two 

 needles being parallel, in the same vertical plane and about 5 cm 

 apart, their poles in opposition. The aluminium support also 

 carried a plane mirror and was suspended in the usual manner 

 by a silken fibre. The astatic pair thus mounted was com- 

 pletely neutral and it was necessary to give it directive force 

 by means of a governing magnet, the position of which was 

 so selected as to give the system a fixed zero point and at the 

 same time a very high degree of delicacy. Deflections were 

 noted by use of a telescope and scale. The figure of merit of 

 the apparatus was determined by substituting a coil of direct 

 windings and known area, for the differentially wound coil, and 

 reading the deflection due to '00001 amperes of current. 



Since, for the effect in question, the figure of merit was the 

 same as though the revolving coil also were directly wound, it 

 could then be derived from a comparison of the total areas of 

 the two coils. 



