y 
The Magnetization of Iron in Bulk. 621 
change in magnetization was then registered by the induced 
voltage i in this secondary circuit, and “the absolute accuracy 
of the result was made fo depend only on the standardization 
of a voltage and the speed of the record. [For let a voltage 
e be induced during time ¢ in a coil with n turns. The 
magnetic movement in the core which has produced this is then 
n=|° . dt, 
nN 
And if @ be the mean voltage during the total time T of 
change 
corresponding, where a is the cross-sectional area of the core, 
to a change in flux density of 
peer 
na 
The immediate object of these tests was to measure directly 
the leakage coefficients of large machines. Incidentally the 
permeabilities of various parts of the magnetic circuits were 
also determined. It was then suggested by Mr. C. C. 
Hawkins that since these were values for iron in bulk, the 
same method might be used for testing large rings specially 
wound for the purpose. 
The lower limit to the size of ring depends only on the 
point at which the iron can be considered to be in bulk. So 
far as this alone is concerned a mass of anything over 
100 lb. would probably be sufficient, in the shape of a yoke- 
ring of the usual proportions; and it is evident that to avoid 
expense the smallest size should be used, provided that the 
method of testing to be employed can be applied with apcumey 
and convenience. 
In the experiments previously referred to the magnet core 
was of cast steel 15 inches diameter, the whole frame weigh- 
ing several tons. This circuit had an inductance of about 
50 henries and a resistance of 10 ohms, the time-constant 
therefore being 5 seconds. 
The method of testing adopted in this case was to make 
the exciting circuit, allow the current to become steady, 
and then to short-circuit the windings, recording photo- 
graphically the voltage induced in the exploring coil. It 
was found that the methods were uniformly accurate, and in 
Phil. Mag. 8. 6. Vol. 8. No. 47. Nov. 1904. 2X 
