532 
MR. T. GRAY ON THE MEASUREMENT OF 
to be accurately measured by means of any ordinary form of ballistic galvanometer 
joined in circuit with a second, or induction, coil surrounding the iron. 
It occurred to me, somewhat more than a year ago, that the cause of the difficulty 
in the ordinary method of measuring magnetic quality in large masses of iron might 
be made an advantage if the method of experiment were changed, that, in fact, the 
desirefl information might be more easily obtained by recording, not the total value 
of the induction for each of several particular values of the magnetizing force, but the 
curve showing the rise of current with time, immediately after the circuit is closed. 
Preliminary experiments showed that, for many of the cases most difficult by the 
ordinary method, this could be done both accurately and easily. In these preliminary 
experiments the coils of a large electromagnet, having laminated cores and pole pieces, 
were joined in circuit with a storage battery and a non-inductive resistance. An auto¬ 
matic arrangement was devised, by means of which the circuit could be closed and 
the instant of closing recorded by a chronograph. The ends of the non-inductive 
resistance were at the same time connected to the quadrants of an electrometer. At 
any desired interval after the closing of the circuit the electrometer could be discon¬ 
nected from the resistance, and the instant of disconnecting recorded by the chrono¬ 
graph. The deflection of the electrometer needle then showed the difference of 
potential between the ends of the non-inductive resistance at the instant when it was 
disconnected. By this means a sufficient number of points on the curve of rise of 
current could be observed to enable it to be drawn. The results obtained in these 
early experiments agree with those given in this paper, and the method may be found 
useful in cases where the rise of current is too rapid to allow of the successful opera¬ 
tion of the method since used. It is evident that a modification of this method with 
a ballistic galvanometer substituted for the electrometer could be easily arranged to 
give satisfactory results. The ordinary ballistic galvanometer and induction coil 
method can also be used with a break circuit arrangement, allowing the total time 
integral to be taken by steps. This method of experiment was not used after the 
preliminary results for which it was devised had been obtained. It was abandoned, 
partly because of its considerable complication, but mainly because a considerable 
number of separate experiments are required to obtain the complete curve, thus 
involving a corresponding number of independent magnetizations of the magnet. 
The arrangement of the apparatus used in the experiments, the results of which 
are here illustrated, is shown in the annexed diagram (fig. 1). In this diagram M is 
the electromagnet experimented on ; G, a commutator for reversing the battery, or, 
when the connection to it was as shown by the dotted lines, for simultaneously cutting 
out the battery and short-circuiting the magnet coils; B is the battery, and R a 
resistance of about one ohm in its circuit to prevent excessive current, when, by the 
operation of the commutator, it is for an instant short-circuited; (? is a mirror galva¬ 
nometer, nearly “dead beat” and of short period; L, the galvanometer lamp; Ch, a 
chronograph, and S, a key for closing the circuit. The drum of the chronograph is 
