ELECTRICAL MEASUREMENT. 247 



with physical theory. It seems that the more accurate an experiment 

 up to the present time, the more nearly does the result approach to 

 being equal to the velocity of light, but still we must hold opinion 

 in reserve before we can say that. The result has to be much closer 

 than it is shown by the experiments already made. But you can all 

 see by the mere mention of such a subject how intensely interesting 

 the pursuing of these investigations further must be. I believe 

 Maxwell at present is making a measurement of this kind on a 

 different plan from any that have been yet made. I have now spoken 

 too long, or I should have described something of the methods already 

 followed in this department, but they are already fully published, and 

 can easily be referred to. 



Now with respect to accurate measurement theory was left far be- 

 hind by practice, and I need not to be reminded by the presence of our 

 President how very much more accurate were the measurements of 

 resistance in the practical telegraphy of Dr. Werner Siemens and his 

 brother, our President, than in any laboratory of theoretical science. 

 When in the laboratory of theoretical science it had not been discovered 

 that the conductivity of different specimens of copper differed at all, in 

 practical telegraphy workshops they were found to differ by from thirty to 

 forty per cent. When differences amounting to so much were overlooked 

 when their very existence was not known to scientific electricians, the 

 great founders of accurate measurement in telegraphy were establishing 

 the standards of resistance accurate to one-tenth per cent. Dr. Werner 

 Siemens and our President were among the first to give accurate 

 standards of resistance, and the very first to give an accurate system of 

 units founded upon those standards. This Siemens unit is still well 

 known, and many of the most important measurements in connection 

 with submarine cables are stated in terms of that unit. By a coinci- 

 dence, which in one respect is a happy one, although there is something 

 to be said on the other side, the unit adopted by Messrs. Siemens 

 founded on the measurement of a certain column of mercury produces 

 and reproduces in the accurate resistance coils the Siemens unit ap- 

 proaches somewhat nearly to the unit which in Weber's system 

 would be io 10 or a thousand million centimetres per second. This is so 

 far convenient that measurements in Siemens units are very easily 

 reduced to the absolute measure. The committee of the British 



