252 Dr. A. Schuster on Unilateral Conductivity. 



above the rank of an irregularity ; that is to say, I am not able 

 to produce it at my own will, although when it is present I am 

 generally able to destroy it. My experiments, however, leave 

 no doubt as to the facts, and they show clearly that, in a circuit 

 composed entirely of copper wires, joined together by means of 

 binding-screws, the electric conductivity may be different in 

 opposite directions. It would be difficult to discover such a 

 difference in the resistance by means of the ordinary ways of 

 measuring it. The changes in the electromotive force of the 

 battery and in the resistance of the wire, through an alteration 

 of temperature or other accidental causes, would be sufficient to 

 mask the effect. If we use, however, the electromotive force of 

 a moving magnet, we are sure that it is always constant as long 

 as the strength of the magnet does not vary and the magnet 

 moves always between certain limits. A magnet rotating rapidly 

 within a coil of wires induces currents in alternate directions in 

 the coil. We are perfectly sure that the electromotive force 

 producing these currents is the same in both directions; and if 

 we can detect any difference in the strength of the currents 

 going in opposite directions through the wire, we may be sure 

 that only a difference in the resistance can produce such a result. 

 I have calculated the effect on the galvanometer-needle of in- 

 duction-shocks following each other in alternate directions at 

 regular intervals of time. If the galvanometer is provided with 

 a clamping arrangement, a final condition will be arrived at in 

 which the galvanometer-needle swings between certain limits. 

 These limits decrease as the interval between the induction- 

 shocks decreases. If, therefore, the rotation of the magnet is 

 rapid enough, the effect of the induced currents on the galvano- 

 meter ceases to be visible. It should, however, be remembered 

 that, although the limits between which the galvanometer-needle 

 moves approach zero, the velocity of the needle remains finite. 

 This, of course, is only true if the two induction-shocks are of 

 equal strength. If the induction-current in one direction is 

 stronger than the current in the opposite direction, the galvano- 

 meter will show a permanent deflection. As we have two strong 

 currents balancing each other, a very small difference in the 

 resistance will have a strong effect. 



II. Description of Apparatus. 



The magnet which was used as electromotive force was fixed 



to the plate of a siren, which could be set into motion by means 



of a pair of bellows. The same instrument has been formerly 



used by R. Kohlrausch and W. Weber *, and later by Kohl- 



* " Electrodynamic Measurements, with special reference to the reduc- 

 tion of intensity to absolute measure," Proc. of the Royal Saxonian Society 

 of Sciences, vol. iii. 





