988 
PROFESSOR J. A. EWING AND MISS H. G. KLAASSEN 
once reduced in amount and reversed in sign.* By giving Bg a series of values, each 
movement of the key to the left determines a new point on the curve, while each 
movement to the right brings back the magnetization to its extreme value. In this 
way as many points are found as may be required to define that part of the curve 
which lies between the zero of magnetizing force and its extreme negative value. To 
find points lying between the positive extreme and the zero of magnetizing force, a 
different process is adopted. The key is kept to the left, and a sudden reduction of 
the current (without change of sign) is effected by withdrawing a i^lug from the 
resistance box II3. This gives a point in the first quadrant of the descending curve, 
and a series of such points are found by selecting successive values of B,, the magne¬ 
tization of the sample being, however, restored to its initial full value before each such 
step. 
To make the matter plainer, we may describe the operation of taking a cycle in 
detail. First, the strength of the magnetizing current was adjusted by selecting a 
proper number of cells (a storage battery was used throughout) and by means of the 
resistance B^, so that the limits of magnetization should have the desired value. At 
this stage there was no resistance in Bo, and the short-circuit plug between e and d 
was in its place. The key K was now rocked back and forth many times to bring the 
magnetic changes into a thoroughly cyclic state—a precaution especially necessary' 
when only a weak degree of magnetization was in question. The ballistic effect of 
this reversal was observed, several times over. Then, with the key K leaning to the 
right, a small sudden reduction of current was made by removing a plug from B3, and 
the ballistic effect of this was observed. The plug in B^ was replaced, the key K 
rocked back and forth to put the metal through a cycle, and then a second resistance, 
larger than the first, was suddenly opened in B^. Operating in this way, a series of 
points were found for values of the magnetizing force H, lying between the positive 
maximum and zero. To continue the curve beyond the axis into the region of 
negative H, the resistance Bg was adjusted to have, first, a rather high value. The 
key was then rocked from right to left, the ballistic effect being read; then this 
resistance was short-circuited ; the specimen was taken through its full cycle by 
rocking the key; Bg was set to be less than before, and its short-circuit plug was 
removed, the key was again rocked to the left, and so on. The curve determined by 
these successive operations was the curve ABCD (fig. 2), in which (ordinates being 
magnetization and abscissae magnetizing force) A is the extreme state, always reverted 
to between each operation and the next; AB is the position determined by plugging 
out resistances in B3, and BCD by rocking the key K with successive (lessening) values 
of Bg. This curve completely defines the magnetic cycle, inasmuch as the return limb 
* Since this description was written, a paper has been communicated to the Royal Society by 
Dr. John Hophinson and Messrs. Lydall and Wilson, in which ballistic experiments are described where 
the method was essentially the same as that used by us. A single movement of a key reversed the 
magnetizing current and introduced resistance. (‘Roy. Soc. Pro.,’ vol. 53, p. 352.) 
