ON THE ABSOLUTE UNIT OF ELECTEICAL RESISTANCE 155 



the square of time enter, besides other quantities which are difficult to 

 determine. The instruments are defective, because the earth-inductor 

 was of such poor proportion and made of such large wire that its 

 average radius was difficult to determine, and was undoubtedly over- 

 estimated. 



It seems probable that a paper scale, which expands and contracts 

 with the weather was used. And lastly, the results with this inductor 

 and by this method have twice given greater results than anybody else 

 has ever found, and greater than the known values of the mechanical 

 equivalent of heat would indicate. 



The latest experiments on resistance have been made by Lorenz of 

 Copenhagen, 10 by a new method of his own, or rather by an application 

 of an experiment of Faraday's. It consists in measuring the difference 

 of potential between the centre and edge of a disc in rapid rotation 

 in a field of known magnetic intensity. 



A lengthy criticism of this experiment is not needed, seeing that it 

 was made more to illustrate the method than to give a new value to 

 the Ohm. The quantity primarily determined by the experiment was 

 the absolute resistance of mercury, and the Ohm will have various 

 values according to the different values which we assume for the resist- 

 ance of mercury in Ohms. 



One of the principal defects of the experiment is the large ratio 

 between the radius of the revolving disc and the coil in which it 

 revolved. 



In conclusion I give the following table of results, reduced as nearly 



as possible to the absolute value of the Ohm in earth q uad \" 



sec. 



iPogg. Ann., Bd. cxlix, (1873), p. 251. 



11 Since this was written, a new determination has been made by H. F. Weber, of 

 Zurich, in which the different results agree with great accuracy. The result has 

 been expressed in Siemen's units, and the comparison seems to have been made 

 simply with a set of resistance coils and not with standards. The modern Siemen's 

 units seem to be reasonably exact, but from the table published by the British 

 Association Committee in 1864, it seems that at that time there was uncertainty as 



to its value. He obtains 1 8. U. = -9550 ---', which is greater or less than 



sec. 



the British Association determination, according as we take the different ratios of 

 the Siemen's to the British Association unit, ranging from -14 per cent above to 1-92 

 per cent below. In any case the result agrees reasonably well with my own. The 

 apparatus used does not seem to have been of the best, and the exact details are not 

 given. But wooden coils to wind the wire on seem to have been used, which should 

 immediately condemn the experiment where a pair of coils is used, seeing that in 

 that case the constant, both of magnetic effect and of induction, depend on the dis- 

 tance of the coils. It is unfortunate that sufficient details are not given for me to 

 enter into a criticism of the experiment. 



