THE HISTORY OF OHM'S LAW 603 



ment that Cavendish's fourth experiment " is the first experimental 

 proof of what is now known as Ohm's law," must be taken with some 

 reservation. The conclusion one reaches is that Cavendish tacitly 

 assumes part II. of the law. It is true that in 1775, two years after 

 the above experiments, he states the law for the combination of resist- 

 ances in parallel and in series though he does not state how he arrived 

 at it, nor does he give any experimental data in proof of his statement. 

 It can therefore hardly be regarded as part of his experimental proof of 

 Ohm's law. "With respect to the effect of cross-section on resistance 

 Cavendish's only recorded experiment consists of a comparison of the 

 shock received through nine small tubes with that received through one 

 large tube of equivalent section and the same length. The fact that the 

 two shocks were equal does not settle the relation between resistance and 

 cross-section except for the case of round conductors. Ohm expanded 

 the work to include sections of other shape. It would seem to be clear 

 that Cavendish can not be credited with the establishment of both parts 

 of the law, and strictly speaking it is an error to speak of him as the 

 " discoverer " of Ohm's law. The most significant obstacle in the way 

 of his doing this was, no doubt, the fact that no such thing as a steady 

 current had as yet been discovered. 



Subsequent to these, as yet unknown, experiments of Cavendish, but 

 before the discovery of steady currents, work on the conductivity of 

 different metal wires was undertaken by Van Marum, Priestley, Children 

 and Harris. Using as thev did the static discharge as a source of cur- 

 rent, their work shows no advance over that of Cavendish either iit 

 results or in method. Peter Barlow, of England, was perhaps -the first 

 to attempt to use a steady current in the study of resistance. He did 

 this by placing successive wires between the terminals of the same 

 voltaic pile, determining the current strength from the tangent of the 

 angle of deflection of the needle of a "multiplier" (galvanometer). 

 The conclusion that he reached was that the resistance of a conductor is 

 directly proportional to the square-root of the length and inversely pro- 

 portional to the cross-section. In looking over the data of these experi- 

 ments one finds discrepancies of 6° to 7° between the observed and 

 calculated deflections based on Ohm's law. This makes it possible to 

 estimate the resistance of the pile, which ought to have been, but was 

 not, included in considering the resistance of the circuit. Such an 

 examination of the data leads to the conclusion that Barlow's failure 

 to reach correct results was due to this neglect of the resistance of his 

 source of current. Had he included this he might have anticipated 

 Ohm, at least to the extent that Cavendish did. 



Cumming used the thermoelectric instead of the voltaic pile as a 

 source of current, otherwise his experiments parallel Barlow's, including 

 the same mistakes and reaching the same erroneous conclusions. 



Davy: We now come to the first experimenter using steady currents 



