76 POPULAR LECTURES AND ADDRESSES. 



coils and ohms, and standard condensers and micro- 

 farads, had been for ten years familiar to the 

 electricians of the submarine-cable factories and 

 testing-stations, before anything that could be called 

 electric measurement had come to be regularly 

 practised in most of the scientific laboratories of 

 the world. I doubt whether, ten years ago, a single 

 scientific-instrument maker or seller could have 

 told his customers whether the specific conductivity 

 of his galvanometer coils was anything within 60 

 per cent, of that of pure copper ; and I doubt 

 whether the resistances of one in a hundred of the 

 coils of electro-magnets, galvanometers, and other 

 electro-magnetic apparatus, in the universities, and 

 laboratories, and lecture establishments of the world, 

 were known to the learned professors whose duty it 

 was to explain their properties, and to teach their 

 use to students and pupils. But we have changed 

 all that ; and now we know the resistances of our 

 electro-magnetic coils, generally speaking, better 

 than we know their lengths ; and our least ad- 

 vanced students in physical laboratories are quite 

 able to measure resistances through a somewhat 



