ON THE MAGNETIC QUALITIES OF IRON. 
1023 
and certain, but its absolute graduations furnished a convenient means of estimating 
the intensity of magnetization from the amount of back electromotive force. 
The heating due to magnetic reversals in the active ring was determined by seeing 
what strength of steady current in the core of the inactive ring would maintain a balance 
in the thermo-electric circuit. The heating current was adjusted, by means of so 
that, after both rings had been brought to the same temperature, the two temperatures 
remained equal while both rose. The thermo-electric junctions were connected to 
oppose each other through a sensitive galvanometer, but an alternative connection 
could be made in which one junction only acted on the galvanometer, so that the 
temperature of the ring might be estimated at any stage in the experiment. It was 
necessary, in fact, to record the temperature of the ring from time to time, to allow 
for chanx^e in the resistance of the iron wire forminef its core. 
The following will serve as an example of the readings taken. It refers to an expe¬ 
riment upon a pair of precisely similar rings, one of which was the Ring VII. of the 
tests already described. The core consisted of iron wire, insulated with cotton. The 
frequency in this experiment was 133 periods per second, and the greatest value of B 
was about 7500. 
(a.) With secondary open :— 
Heating current, in amperes. 
Reading of 
Cardew voltmeter. 
Resistance of 
iron core. 
O'75 (too strong) 
22 
5-85 
O'65 (too weak) 
22 
to 
0'7 (balance) 
22 
5'90 
(b.) With secondary closed through a low resistance ;— 
Heating current, in amperes. 
Reading of 
Cardew voltmeter. 
Resistance of 
iron core. 
O'75 (too strong) 
24i 
5'80 
0'65 (too weak) 
to 
to 
O'7 (balance) 
24 
5'90 
The reading of the microscope hot-wire voltmeter was the same in both parts of the 
experiment, the difference in the Cardew readings being due to the greater volume of 
magnetizing current that was required when the secondary circuit was closed. It will 
be seen that no difference could be detected in the amount of balancing current neces¬ 
sary to maintain equality of temperature. Other experiments gave confirmatory and 
quite conclusive results. One more may be quoted :— 
Frequency 69 periods per second. Greatest value of B about 13500. 
(a.) Secondary open. Balancing current 236 on arbitrary scale. 
(5.) Secondary short-circuited. Balance with current between 231 and 244. 
