In January 1882, Mr. Herbert Tomlinson communicated to the Royal Society[41] the results of experiments on a number of electric conductors. The change of conductivity by the application of stress was found to be excessively small. For carbon it was less than one-thousandth part of one per cent. for an increase of fifteen lbs. on the square inch in the pressure. For iron it was slightly greater, and for lead nearly twice as great, but with all other metals less. If this alleged property were the one on which the action of telephone transmitters depended, then lead ought to be twice as good a substance as graphite; whereas it is not nearly so good. Professor W. F. Barrett, in 1879,[42] made some experiments on the buttons of compressed lamp-black used in Edison’s transmitter, and found that when an intimate contact was satisfactorily secured at the beginning, “pressure makes no change in the resistance.” In the face of all this precise evidence, it is impossible to maintain the theory that the electric resistance of plumbago or of any other such conductor varies under pressure. The only person who has seriously spoken in favour of the theory is Professor T. C. Mendenhall, but in his experiments he took no precautions against variability of contacts, so that his conclusions are invalid. More recently still, Mr. O. Heaviside and Mr. Shelford Bidwell have experimented on the variations of resistance at points of contact.[43] Mr. Heaviside’s experiments were confined to contacts between pieces of carbon, and though extremely interesting as showing that the resistance of such contacts are not the same, even under constant pressure, when currents of different strength are flowing, do not throw much light on the general question, because they leave out the parallel case of the metals. Mr. Bidwell’s very careful researches were chiefly confined to carbon and bismuth. The choice is unfortunate, because bismuth the most fusible and worst conductor amongst metals (save only quicksilver) is the one metal _least_ suited for use in a telephone transmitter. Mr. Bidwell’s conclusions, so far as they are comparative between carbon and “the metals,” are therefore necessarily incomplete.