ELECTRICAL UNITS OF MEASUREMENT. 77 



wide range with considerable accuracy. I should 

 think, indeed, that w r ith the appliances in ordinary 

 use, they are more likely to measure resistances of 

 from 100 to 10,000 ohms to an accuracy of T l per 

 cent., than they are to be right to one millimetre in a 

 metre in their measurements of length. It certainly 

 is a very surprising result that in such a recondite 

 phenomenon such a subtle quality to deal with 

 as electric resistance, which is so very difficult to 

 define, and which we are going to learn is a velocity, 

 every clerk in a telegraph station, the junior 

 students and assistants in laboratories, and even 

 workmen in electric lighting establishments, are 

 perfectly ready to measure, more accurately than 

 you would measure the length of ten feet of wire, 

 the resistance of electric conductors in definite 

 absolute units. 



I suppose, too, nearly every apparatus-room and 

 physical laboratory possesses a micro-farad, but 1 

 am afraid its pedigree is not often known ; and if 

 its accuracy within 10 per cent, were challenged, I 

 doubt whether, in many cases, any one, whether 

 maker, or possessor, or other electrical expert, 



