402 Mr. Gr. Chrystal on 131- and Unilateral 



sistance of the circuit depends on the intensity of the current, 

 one of the currents will prevail, and the presence of the alter- 

 nating currents will affect the permanent deflection due to the 

 small constant current, whereas, if Ohm's law were true, 

 there would be no such effect. Without discussing Dr. 

 Schuster's results here, we may remark that a similar test of 

 Ohm's law could be obtained by merely passing through a 

 galvanometer the currents from the secondary coil of an in- 

 ductorium. It is well known that if the primary be made and 

 broken periodically, there will be an alternating current in the 

 secondary, which will have the same period. The whole 

 quantity of electricity which passes during a complete oscilla- 

 tion is zero ; but the maximum intensities of the positive and 

 negative parts of the current are very different. The positive 

 part, starting immediately after the break, has a considerable 

 initial intensity, which is independent of the resistance of the 

 secondary ; the negative part, starting at the make, begins, on 

 the contrary, with zero intensity, and never reaches so large 

 a maximum as the positive *. 



It follows that if the resistance of the circuit depends on the 

 intensity of the current, then the two parts of the current will 

 not experience equal resistances, and we shall get a galvano- 

 meter indication in the direction of that which has the advan- 

 tage. Any such effect would be much increased by the in- 

 troduction into the secondary circuit of a resistance composed 

 of very fine wire. It is easy enough to calculate what this 

 resistance should be, in order to produce the greatest effect on 

 the galvanometer. 



Such a resistance I used in the shape of a fine German- 

 silver wire (*002 inch diameter) wound on a cylindrical piece 

 of vulcanite about 9 inches long, the turns being insulated 

 from each other by the thread of a screw of one hundred turns 

 to the inch cut in the vulcanite. The whole is enclosed in a 

 glass tube with brass caps and copper terminals. For this in- 

 strument I am indebted to Mr. Garnett, of St. John's, De- 

 monstrator at the Cavendish Laboratory. The induction-coil 

 used was of the ordinary lecture-room form by Apps ; the 

 primary was made and broken by electric tuning-forks of 

 various pitch. 



The results I obtained indicated an apparent departure 

 from Ohm's law, sometimes in one direction, sometimes in the 

 other. The presence or absence of the fine wire in the circuit 

 did not seem to be an essential condition of the phenomenon. 



* This supposes the period of alternation long compared with the time- 

 constants of the coil; the same description applies, in a modified degree, 

 to other cases. 



