290 H. A. Rowland—Absolute Unit of Electrical Resistance. 
7 yz, and as A=$ nearly, we have 
-¥ —1.0008, 
which would diminish the value of the resistance by ‘16 per 
ce 
n 
As the time we have allowed for turning the earth-inductor 
is probably greater than it actually was, the actual correction 
will be less than this. 
The correction for the extra current induced in the inductor 
and galvanometer, as given by Maxwell’s equation,* has been 
shown by Stoletow to be too small to affect the result appre- 
ciably. : 
We may sum up our criticism of this experiment in a few 
words. The method is defective because, although absolute 
resistance has the dimensions of “F“, yet in this method the 
fourth power of space and the square of time enter, besides 
other quantities which are difficult to determine. The instru- 
ments are defective, because the eurth-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,+ by a new method of his own, or rather 
by an application of an experiment of Faraday’s. It consists 
im measuring the difference of potential between the center 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 
which it revolved. 
* “ Electricity and Magnetism,” Art. 762. 
t Pogg. Ann., Bd. cxlix, (1873), p. 251. 
