ON OHM^S LAW. 37 



The difficulty of testing tliis law arises from the fact that the cinrcnt 

 generates heat and alters the temperature of the couductor, so that it is ex- 

 tremely difficult to ensure that the conductor is at the ^ame temperature 

 when currents of different strengths are passed through it. 



Since the resistance of a conductor is the same in whichever direction the 

 current passes through it, the resistance, if it is not constant, must depend 

 upon even powers of the intensity of the current through each element of 

 the conductor. Hence if we can cause a current to pass in succession 

 through two conductors of different sections, the deviations from Ohm's law 

 will bo greater in the conductor of smaller section ; and if the resistances of 

 the conductors are equal for small currents, they will be no longer equal for 

 large currents. 



The first method which occurred to the Committee was to prepare a set of 

 five resistance-coils of such a kind that their resistance could be very accu- 

 rately measured. Mr. Hockin, who has had great experience in measuring 

 resistance, suggested 30 ohms as a convenient magnitude of the resistance 

 to be measured. The five coils and two others to complete the bridge were 

 therefore constructed, each of 30 ohms, by Messrs. Warden, Muirhead, and 

 Clark, and it was found that a difference of one in four millions in the ratio 

 of the resistance of two such coils could be detected. 



According to Ohm's law, the resistance of a system consisting of four 

 equal resistance-coils joiued in two series of two should be equal to that of 

 any one of the coils. The current in the single coil is, however, of double 

 the intensity of that in any one of the four coils. Hence if Ohm's law is not 

 true, and if the five coils when compared in pairs with the same current arc 

 found to have equal resistances, the resistance of the four coils combined 

 would no longer be equal to that of a single coil. 



A system of mercury-cups was arranged so that when the sj^stem of five 

 coils was placed with its electrodes in the cups, any one of the coils might be 

 compared with the other four combined two and two. After this comparison 

 had been made, the system of five coils was moved forward a fifth of a revolu- 

 tion, so as to compare the second coil with a combination of the other four, 

 and so on. 



The experiments were conducted in the Cavendish Laboratory by Mr. G. 

 Chrystal, B.A., Fellow of Corpus Christi College, who has prepared a report 

 on the experiments and their results. 



A very small apparent deviation from Ohm's law was observed ; but as this 

 result was not confirmed by the much more searching method of experiment 

 afterwards adopted, it must be regarded as the result of some irregularity in 

 the conducting-power of the connexions. 



The defect of this method of experiment is that it is impossible to pass a 

 current of great intensity through a conductor without heating it so rapidly 

 that there is no time to make an observation before its resistance has been 

 considerably increased by the rise of temperature. 



A second method was therefore adopted, in which the resistances were com- 

 pared by means of strong and weak currents, which were passed alternately 

 through the wires many times in a second. The resistances to be compared 

 were those of a very fine and short wire enclosed in a glass tube, and a long 

 thick wire of nearly the same resistance. When the same current was passed 

 through both wires, its intensity was many times greater in the thin wire 

 than in the thick wire, so that the deviation, if any, from Ohm's law would 

 be much greater in the thin wire than in the thick one. 



Hence, if these two wires are combined with two equal large resistances in 



