ON THE xMEASUREMENT OF MAGNETIC HYSTERESIS. 
57 
Since the spring connected with e does not leave any stud before it touches the 
next one, as the arm is moved from one side to the other, the primary circuit is 
never broken except possibly in the central position of the arm. The circuit will be 
broken for an instant ixi the central position ‘unless the spring connected with e 
touches the two central studs at the same instant that the spring connected with f 
touches both the sectors c, d. It is more or less a matter of chance whether this 
break of the circuit occurs, but if it does occur it is only after the current has been 
reduced to a small value by the introduction of the large resistance of 240 ohms. 
Except for this uncertainty we may say that the current is reversed in several steps 
which (except possibly those occurring in the uncertain part of the motion) are not 
sudden because of the great self-induction of the choking coil (§ 33). Though (with 
a possible exception) there is no sudden change in the current, the rate of variation of 
the current is no doubt much greater at some stages of the change than at others. 
The key generally worked well; it was, however, subject to slight uncertainties. 
§ 32. A fifth key was designed in 1900. Our aim was to ensure that the primary 
circuit should never be broken, and also that the resistances introduced into the 
circuit should be as small as possible, so that the rate of change of the current should, 
at every part of its variation, be as small as possible. 
The battery is connected through the Weston ampere-meter, WA (fig. 5), v/ith the 
terminals g, h on an ebonite arm working about a pivot at 0. The terminals p, h are 
connected to two brass springs, h, I, one at. each end of the arm. These springs slide 
over a series of sectors a, h, c, d, e, cut out of a brass ring. Tlie sectors a and h and 
VOL. CXCVIII.-A. 
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