Galvanometer Deflection. 403 



I was therefore led to suspect that the cause lav in the galva- 

 nometer itself — a suspicion which became certainty when I 

 found that reversing the galvanometer-connexions with the 

 secondary, or reversing the primary, had no effect whatever 

 on the character of the phenomena. It appeared that the 

 effects observed could be analyzed as follows : — Suppose we 

 are using a Thomson's galvanometer with mirror, lamp, and 

 scale as usual, and let the scale be placed parallel to the coil- 

 windings, a common perpendicular passing through the centre 

 of the mirror, and the slit through which the light comes from 

 the lamp ; then two distinct states appear, according to the 

 relation between the strength of the alternating currents in 

 the secondary and the strength of the magnetic field in the 

 axis of the coil due to the earth and deflecting magnet. 1st. 

 If the currents are powerful enough and the magnetic field 

 weak enough, the spot of light goes off the scale completely, 

 either to one side or the other, and remains there. It can be 

 made to go to either side and remain there by starting it off 

 properly, which is easily enough managed by throwing on the 

 alternating currents after it has passed the zero towards the 

 side to which it is desired to send it. The spot will not re- 

 main at zero, even when placed there very carefully. This 

 phenomenon I call bilateral deflection* . 2nd. If the strength 

 of the currents be decreased sufficiently, whether by interpola- 

 ting resistance in the secondary or primary, or by reducing the 

 electromotive force in the primary, or by shunting the gal- 

 vanometer — or if, on the other hand, the strength of the 

 magnetic field be sufficiently increased, say, by lowering the 

 deflecting magnet. — 



I. If the spot of light be brought, when there are no elec- 

 trical oscillations, to zero on the scale, then on setting the coil 

 in action it comes to rest at zero and remains steady there. 



II. If the spot be brought to any position right or left of 

 zero, then when the coil is in action it comes to rest in some 

 position a little further to the right or left respectively, and 

 remains steady there. The difference between these positions 

 is greater the greater the original deflection of the spot from 

 zero. This phenomenon I call unilateral deflection. 



IIL No difference of any kind was produced in any of 

 these phenomena by reversing the connexions of the secon- 

 dary with the galvanometer. Xor did the character of the 

 phenomenon depend on the number of alternations per 

 second, which in my experiments varied from 10 to 200. 

 It was the second of the last-mentioned set of phenomena 



* After Pog-gendorff, who originally observed the phenomenon and called 

 it " Doppelsinnige Ablenkung.'" — Pogg. Ann. vol. xlv. p. 353 (1838). 



21)2 



