Page:Popular Science Monthly Volume 83.djvu/607

Rh 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 11. of the law. It is true that in 1775, two years after the above experiments, he states the law for the combination of resistances 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 they did the static discharge as a source of current, their work shows no advance over that of Cavendish either in 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 proportional to the cross-section. In looking over the data of these experiments 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