ELECTRICAL UNITS OF MEASUREMENT. 1 1 1 



That is a thing that can be done from the be- 

 ginning, with nothing but cutting tools and pieces 

 of metal to begin with. Let him get a piece of 

 brass and make a wheel, and cut it to two 

 thousand teeth. I do not know how many teeth 

 Foucault used, but our traveller can go through 

 the whole process, and set the wheel revolving 

 at some uniform rate (not a known rate, because 

 he has no reckoning of time) ; and he will tell 

 what the velocity of the wheel is in terms of the 

 velocity of light, which is known to be about 

 300,000 kilometres per second. If he is electric- 

 ally minded, as this evening we are bound to 

 suppose our scientific traveller to be, he will 

 think of "v" or of an ohm. He may make a 

 Siemens unit ; that he can do, because he has 

 his centimetre, and he finds mercury and glass 

 everywhere. Then he goes through all that Lord 

 Rayleigh and Mrs. Sidgwick have done. He will, 

 with a temporary chronometer or vibrator, obtain 

 a provisional reckoning of time, and he will go 

 through the whole process of measuring the resist- 

 ance of a Siemens unit in absolute measure, 



