Relations between Electrical Measurements. 515 



render it practically unavailable for any exact determination. 

 The apparent effect of absorption is to cause r } the resistance of 

 the material, to be a quantity variable with the time t, and the 

 laws of the variation are very imperfectly known. 



46. Experimental Determination of the Ratio, v, between Elec- 

 tromagnetic and Electrostatic Measures of Quantity. — In order 

 to obtain the value of v, it is necessary and sufficient that we 

 should obtain a common electrostatic and electromagnetic mea- 

 sure of some one quantity, current, resistance, electromotive 

 force, or capacity. There are thus five known methods by which 

 the value can be obtained. 



(1 ) By a common measure of quantity. Let a condenser of 

 known capacity, s, be prepared (40). Let it be charged to a 

 given potential P (47). Then the quantity in the condenser will 

 be sV in electrostatic measure. The charge can next be mea- 

 sured by discharge through a galvanometer (25) in electromag- 

 netic measure. The ratio between the two numbers will give 

 the value of v. The only difficulty in this method consists in 

 the measurement of the potential P entailing the measurement 

 of an absolute force between two electrified bodies. This method 

 was proposed and adopted by Weber*. 



(2) By a comparison of the measure of electromotive force. 

 The electromotive force produced by a battery, in electrostatic 

 measure, can be directly weighed (43). Its electromotive force, 

 in electromagnetic measure, can be obtained from the current it 

 produces in a given resistance (28). The ratio of the two num- 

 bers will give the value of v. This method has been carried out 

 by Professor W. Thomson, who was not, however, at the time 

 in possession of the means of determining accurately either the 

 absolute resistance of his circuit or the absolute value of the 

 current t 



(3) By a common measure of resistance. We know (29 and 

 45) how to measure resistances in electromagnetic and electro- 

 static measure. The ratio between these measures is equal to v 2 . 

 The measure of resistance in electrostatic measure is not as yet 

 susceptible of great accuracy. 



(4) By a comparison of currents. The electromagnetic value 

 of a current produced by a continuous succession of discharges 

 from a condenser of capacity s can be measured (18, 19). The 

 electrostatic value of the current will be known if the potential 

 to which the condenser is charged be known. The ratio of the 

 two numbers is equal to v. 



* Pogg. Ann. August 1856, vol. xcix. p. 10. Abhandlungen der Kbn. 

 Sachsischen Gesellschaft, vol. iii. (1857), p. 266. 



t Paper read before the Royal Society, February 1860. Vide Proceed- 

 ings of the Royal Society, vol. x. p. 319, and Phil. Mag. S. 4. vol. xx. p. 233. 



