Unit of Electrical Resistance. 1 75 



mentioned Professor Thomson, who in a lecture in 1857 to the 

 British Association gave the relative specific resistances of the 

 copper and gutta percha of the Atlantic cable. In a note below 

 is an extract from a printed report published in a local paper*. 

 Next to Professor Thomson I should have mentioned myself, 

 who, acting not under Dr. Siemens's direction, but at the sugges- 

 tion of Professor Thomson, made in the spring and summer of 1859 

 experiments on this subject, more complete, I think, than are 

 contained in Dr. Siemens' s Red-Sea Report. These results were 

 published at the Meeting of the British Association in 1859, and 

 communicated in a more complete form to the Royal Society ; 

 whereas it is not till 1860 that I find any publication by Dr. 

 Siemens on the subject, in a paper read to the British Association 

 in 1860, the very body which had received the two previous com- 

 munications. He is silent as to both of these. He gives less 

 complete results than those given in the previous year ; he gives 

 no experiments on the difference between positive and negative 

 currents, nor any account of the curious effect of electrification 

 due to the time the current has been applied, and without 

 which any record of resistance-measurements is nearly useless. 

 Experiments on both these points are given in my paper. 

 Surely, then, it cannot have been from any hostile feeling to 

 Dr. Siemens that I said nothing about the measurement of the 

 resistance of insulators. 



* Extract from Professor W. Thomson's Lecture before the Members 

 of the British Association at Dublin, 1857, taken from the Glasgow * North 

 British Daily Mail ' of the 4th of September, 1857 :— 



" He had now described the material and the process of manu- 

 facture. He would like to say something of the relative qualities of gutta 

 percha and copper as conductors, for they were both conductors — the dis- 

 tinction between non-conductors and conductors being not an absolute 

 distinction, but only a relative distinction. Gutta percha is not a non- 

 conductor, but a very powerful resister of electricity. Gutta percha and 

 every known substance conducted electricity through it. (The lecturer 

 proceeded to explain that, when tested by the galvanometer, there was 

 very little difference in the force of a current sent into 2500 miles of the 

 Atlantic cable, whether the circuit was or was not completed.) This 

 seemed rather hopeless for telegraphing (he continued), where there was so 

 much leakage that the difference could not be discovered between want 

 of insulation and insulation at the remote end. But if there were 49-50ths 

 lost by defective insulation, it would only make the difference between 

 sending a message in nine minutes instead of in eight. The explanation of 

 this was simple, but must be reserved. He then proceeded to allude to 

 the variations of the conducting-power of gutta percha in different tempe- 

 ratures, and gave several comparisons, the result of experiments by Mr. 

 Whitehouse and himself on this subject, and showed that the variations 

 observed in portions of the same material were caused by difference of tem- 

 perature. At hot temperatures gutta percha resisted twenty million million 

 million times as much as copper ; at cold temperatures one hundred million 

 million million " 



