198 Sheldon — Magneto-optical Generation of Electricity. 



then a magnetic field should result and an electromotive force 

 be induced in a coil surrounding that field. Such a result 

 would be obtained by rotating the polarizing nicol. The 

 rapidity of rotation must be very great, and, if it requires 278 

 amperes (an impressed electromotive force of 2000 volts) to 

 rotate the plane through 360°, then to produce this electromo- 

 tive force the polarizer must be revolved with a frequency of 

 the same order as of the oscillations of light. But a nicol can- 

 not be revolved much above 200 times per second. The cen- 

 trifugal force resulting from a higher rate will, owing to the 

 strain produced, interfere with the performance of its func- 

 tions as a polarizer. This rate of 200 revolutions per second 

 would produce, in the apparatus employed, an electromotive 

 force of perhaps 0,000000001 volts, giving a current too small 

 to be detected by any galvanometer in my laboratory. Hence 

 use was made of the extreme delicacy of the telephone as a 

 substitute, and a swinging of the plane instead of a revolution. 



The arrangement of apparatus was as follows : Light from 

 an arc lamp, after passing through a large nicol, was reflected, 

 at a very obtuse angle, from a small movable mirror and then 

 passed through the bisulphide of carbon in the coil before men- 

 tioned. The two terminals of the coil were carried to a room 

 three stories below and in another part of the building. Here 

 they were connected through a telephone and a switch. The 

 mirror (10x30 mm ) was fixed in a brass frame free to rotate 

 about an axis nearly parallel with the ray of light. This frame 

 was connected by an eccentric and gears to the main shaft in 

 the work shop. By this arrangement the mirror was made to 

 oscillate through 45° about 300 times each second. The plane 

 of polarization was thus twisted through twice that amount, or 

 90°, in the same time. While this oscillation was going on in 

 the workshop, an ear placed at the telephone at the other end 

 of the circuit could easily distinguish a tone, which, however, 

 was the octave above that made by the moving mirror. "When 

 the circuit was broken the sound ceased to be heard, but upon 

 again closing the tone became audible. With a rate of 200 

 oscillations per second the note was not so easily distinguished. 

 But upon closing the circuit that peculiar sizzling noise so com- 

 mon in telephone circuits was heard. 



During the experiments the mirror was frequently broken 

 by the high rate of vibration. But another was quickly sub- 

 stituted by my assistant, Mr. Baker, whom I have to thank for 

 this and the construction and management of the rotating ap- 

 paratus. 



Polytechnic Institute of Brooklyn, 

 June, 1890. 



