336 THE ELECTRIC TELEGRAPH. 



of its poles is deflected (supposing, for the moment, all the 

 magnetism to be collected in the poles) bears a certain rela- 

 tion to the horizontal component M of the earth's magnetism, 

 and thus, in the form of equation (4, is 



F = M tan.0 = 

 whence the intensity 



I = M tan. ^=- 



an expression for which we only want to know the value of 

 M, at the moment of the observation, to give us the intensity, 

 I, in electro-magnetic absolute units. 



With this one electrical magnitude known in absolute 

 measure, we want only one more to enable us to express the 

 third dependent magnitude contained in Ohm's law, and 

 the whole system is in our hands. The magnitude next 

 to I in rank of facility, to be determined, is R. 



To do this for us to find the absolute value of R, 

 has lately been the task of a very diligent committee, com- 

 posed of some of the most expert physicists in England, 

 appointed by the British Association ; and they have ful- 

 filled their commission with much credit to themselves, 

 and benefit to the scientific world. Their results, published 

 in a series of reports on standards of electrical resistance, 

 for which we are principally indebted for our knowledge of 

 the subject, are very comprehensive and valuable. The 

 results, in detail, would be out of place here; but it is 

 necessary to give a glance at the modus operandi, in order to 

 get a clear notion of how the absolute measure of R has been 

 arrived at. 



The simplest way to do this was that followed by Professor 

 Thomson, who determined the absolute resistance of a wire 

 by means of Dr. Joule's experimental measurement of the 

 heat developed in it by the passage of a current. But a 

 more indirect method was followed by the committee with 

 better results, that of measuring the magneto- electric 



