40 



KEPORT 1876. 



seem to show, among otlicr things, that conclusiocis rcBpccting the accuracy 

 of Ohm's law cannot safely be drawn from experiments of the nature of 

 those made by Dr. Schuster. 



FiEST Experiment. 



Suppose that we had five resistance-coils, which, when compared with each 

 other by means of the same current, were equal, say each = E. That is to 

 say, if any two of the resistance-coils were inserted in the branches A B and 

 U D of a Whcatstone's bridge, the other tAvo arms, A C and C D, being two 

 other equal resistances, then the galvanometer G inserted betM'cen E and C 

 would indicate no current. 



Fig. 1. 



Kg. 2. 



Suppose now that we replace the coil E in BD by four of the equal coils 

 arranged in multiple arc, as in fig. 2. Then, if Ohm's law be true (i.e. if 

 resistance be independent of current), if p be the resistance between B and D, 



1 _ ^ J. = 1 

 p 2E 2R E' 



I.e. p=E, and there will still be no deflection in the galvanometer. But if 

 Ohm's law bo not true, and the resistance be a function of the current, then, 

 since the current through A B is nearly the same as in the first experiment, 

 while that through B E D and B F D is half, the resistances in B E, E D, B F, 

 F D will bo no longer eqiial to E, but either greater or less, and the galvano- 

 meter will be deflected. 



Under the direction of Professor Maxwell, part of the funds at the disposal 

 of the Committee were devoted to providing two sets of coils sijecially 

 adapted for the above experiment. One set consisted of five coils of silk- 

 covered German silver wire (diameter -G millim.), each of resistance as nearly 

 as possible equal to 30 B.A. units. These were all wound together in the 

 usual way round one bobbin ; the terminals consisted of ten pieces of stout 

 copper wire, insulated from each other by a ring-shaped piece of ebonite, 

 through which all of them passed. These stout wires were bent over, and 



