66 
MESSRS. G. F. C. SEARLE AND T. G. BEDFORD 
It was only in the later experiments that we were guided by the comjDlete theory 
to test the accuracy of the measurements by varying S, but from the tests just 
described we may conclude that the value of U, obtained from the dynamometer 
throw, was in all our experiments very nearly equal to W. 
Energy dissipated by Eddy Currents. 
§ 42. In our experiments the ratio X/Y was small, so that, Y being small in com¬ 
parison with U, X was small also, and thus U and W were nearly equal. 
We now consider the case in which the section of the specimen is large, so that S 
has to l)e made large and n small, in order to reduce the sensitiveness of the apparatus. 
Under these conditions X/Y is large, while Y is now so small in comparison with U 
that it eludes observation, and thus cannot be determined by varying S, the resistance 
of the secondary circuit. These conditions have prevailed in the experiments made 
1)y Mr. R. L. Wills with some of our apparatus; in these experiments the eddy 
current loss was so large that systematic measurements were made to determine X for 
every value of Hq employed. 
Mr. Wills used the key described in § 32, and found that for a given total 
resistance of the rimary circuit the “throw” of the dynamometer is practically 
independent of the resistances denoted liy D in the description of the key. This key 
divides the primary circuit into two j^ortions; the resistance of the portion which 
includes the battery is denoted liy T, and the resistance of the other portion by R. 
When the voltage E was to be changed in order to make the observations suggested 
in § 15, Mr. Wills changed the total resistance, Pt -p T, of the jirimary circuit, 
mainly by changing Pt. The resistance T was small compared with R, and was used 
as a means of obtaining an exact adjustment of the current to definite values. Xo 
attempt was made to make T bear any fixed ratio to R. 
The reversal of the current does not take place quite in the manner described 
in § 15. In addition to the effect of the resistances D, there is the further jioint of 
difference that although R + T is adjusted to he accurately jiroportional to E, the 
voltage driving the current Cq, yet Pt is not accurately proportional to E because T 
does not bear any fixed ratio to R. The value of dC/dt and, consequently, the power 
absorbed by the eddy currents while the primary current is sinking to zero is thus not 
quite proportional to E for a given value of C. 
During the subsequent rise of the current the wliole resistance R + T comes into 
]day, and, the resistances D being compai'atively small, the value of dC/dt for a given 
value of C is nearly projiortional to E. 
Now the increment of current dC contrilmtes to X a quantity proportional to 
[dB/dJlY . dC/dt. Thus although, for a given value of 0, dC dt is much greater 
during the fall than during the rise of the current, because c/B/c/H for the iron of 
the choking coil is much smaller during tlie first than during the second stage, yet, 
