266 Prof. G. Wiedemann on the Methods 



coils consisting of cotton-covered copper wire about 3^ millim. 

 in thickness, and having a resistance of about 5 ohms — one, the 

 multiplier, surrounds a cylindrical magnetic needle about 

 100 millim. long and 10 millim. thick, fixed in the magnetic 

 east and west plane. The diameter of the coils (interior about 

 960 millim., exterior 1040 millim., and breadth of coils about 

 254 millim.) is so large that the magnetic forces acting on the 

 needle may be supposed, without perceptible error, to be con- 

 centrated at their centre. The other coil, connected with the 

 first by conducting-wires, the inductor, is capable of rotation 

 about a vertical axis, out of the east and west position through 

 an angle of 180°. This displacement of the inductor takes place 

 suddenly at such times that the currents induced in it and 

 transmitted through the multiplier render the deflections of 

 the needle constant according to the " method of multiplica- 

 tion " or the " method of recoil." 



In carrying out the experiments, the displacement of the 

 inductor by means of clockwork was not found to be practi- 

 cable, and it was therefore effected by hand. The time required 

 for the displacement (some 2 seconds) must only be a small 

 fraction of the time of vibration of the magnet hanging in tho 

 multiplier. For this purpose the time of vibration of the 

 needle is increased (up to 30 seconds) by placing the needle 

 in a stirrup to which a tube of brass 272 millim. long was 

 fastened in a horizontal position and at right angles to the 

 needle, the ends of which carried plane mirrors. Telescopes 

 with parallel scales are placed before both mirrors, each at a 

 distance of 4 millim. By this double reading the difficult 

 measurement of the scales from the mirrors is replaced by the 

 easier measurement of the distance of the scales from each 

 other, and of the distance of the mirrors apart. An exami- 

 nation of the point whether, and how far, the instants of dis- 

 placement of the inducing coil may differ from those indicated 

 by calculation, and how far these possible deviations may affect, 

 might be easily made*. In any case, according to the expe- 

 riments which have been made, this influence cannot be of 

 importance, since the results obtained at different times by use 

 of a magnet 100 millim. long agree with each other to within 

 0*06 per cent., and those obtained with a magnet 200 millim. 

 long do not differ at all from the mean result. This shows at 

 the same time that the instrument had not altered in any im- 

 portant respect in the interval. The inductor must be so far 

 removed from the multiplier that the currents in the first 

 cannot directly deflect the needle. 



* Compare Chwolson, Bulletin de St. Petersbourg, vol. ii. p. 403 ; Bei- 

 blatter, vol. v. p. 450. 



