128 Dr. S. W. Richardson on the Magnetic 



is considered, and it is shown how the total induction in the 

 cylinder at any time t after the magnetic force is destroyed 



A 2 



can be calculated if we know the value of — '-— , where u. 



a 



and er are the permeability and specific resistance of the 



material of the cylinder and a is its radius. We can make 



use of this result to form an approximate idea of the speed at 



which the secohmmeter might be driven without materially 



affecting the magnetic measurements. 



The greatest permeability for alloy No. 1, which is the 



most permeable of the three specimens considered in this 



paper, was 680. If we take a as 06 cm. and a as 20,000 



(the temperature being 380° C.) ^^O'lo. A com- 

 parison with the table on p. 357 ( l Recent Researches ') will 

 show that in this case the total induction will have diminished 

 to 1 per cent, of its original amount in 0*1 of a second. 



Hence we see that the number of alternations per second 

 in the experiments on this alloy should be less than 10 

 per second, in order that the results may be relied upon to 

 1 per cent. 



In general the secohmmeter made about 3 alternations per 

 second. It could easily be seen whether, in any given case, 

 the eddy currents were affecting the measurements, by obtain- 

 ing a balance and then allowing the speed of the secohmmeter 

 to increase. 



The effect of eddy currents would be shown by the balance 

 being disturbed. 



Care was taken, during the experiments, that the speed of 

 the secohmmeter should in all cases be less than that at which 

 this occurred. 



§ III. The Determination of the Temperature. 



The resistance of the secondary of the ring (which was of 

 platinum wire and was wound next to the ring) w^as taken as 

 a measure of the temperature. Thick platinum leads were 

 connected to the secondary close to the ring, and after passing 

 out of the box containing it dipped into mercury cups sus- 

 pended in an oil-bath. From these cups copper wires passed 

 to the key K 4 (fig. 3), by means of which the secondary could 

 be connected at will to either the magnetic bridge or the wire 

 bridge. Side by side with the platinum leads were placed 

 compensating leads. These were cut from the -same specimen 

 of wire as that used for the leads to the secondary, and also 

 dipped into mercury cups suspended in the oil-bath. Copper 





