106 Nichols and Franklin — Direction and 



The most serious defect of this apparatus lay in the imper- 

 fect balancing of the differential windings. The position of 

 the needle when a current traversed the coil always differed 

 considerably from that which it assumed when the current was 

 broken, and with one ampere of current the deflection amounted 

 to several centimeters. The difficulty was however not of a 

 nature to interfere altogether with the progress of the inves- 

 tigation and a series of readings w»re accordingly made with 

 coil at rest and in motion. The rate of revolution during the 

 determinations was 380 turns per second. The' direction of the 

 current through the coil and the direction of rotation of the 

 latter were repeatedly reversed while the position of the needle 

 was under observation. The result was an entirely negative 

 one, no measurable effect upon the needle resulting from the 

 motion of the coil. The current traversing the coil was meas- 

 ured upon a Moler's " swinging arm " galvanometer. During 

 a portion of the time it exceeded one ampere. A determina- 

 tion of the figure of merit of the apparatus made immediately 

 after the conclusion of the series of observations gave as a 

 result : 



l mm denection=0-0000164 amperes. 



When one ampere of current traversed the coil, therefore, a 

 change in the apparent magnetic moment of the latter (due to 

 rotation), in the ratio 1 : 1± '0000164 would have shown itself 

 in a change of deflection amounting to l mm . Such a variation 

 could not have escaped notice. 



The mean circumference of the windings of the coil was 

 23"939 cm , so that at 380 revolutions per second the wire had 

 an average velocity, in the direction of its own length, of 

 9096-82 cm . 



If we suppose the current to consist in the movement of 

 electricity along the wire in a given direction, the velocity, 

 relative to the conductor, being the same whether the coil is at 

 rest or in motion, and the deflection of the needle to be due to 

 the translatory movement of electricity with reference to the 

 needle and proportional to that movement, it is easy to calcu- 

 late the change in deflection, for any assumed current-velocity, 

 which will be produced by a given rate of rotation of the coil. 



Let Y w be the linear velocity of the conductor, 

 V c the velocity of the current, 

 Cj the current traversing the conductor, 



C 2 the current necessary to produce a given deflection when 

 traversing a directly wound coil of the same total area. 



V C 



Then V,.=- " 



C„ 



