ON THE MAGNETIC QUALITIES OF IRON. 987 
advantage that each point in the cycle is reached by a single step from the end 
of the cycle. 
The diagram (fig. 1) shows the arrangement used. 
Fig. 1. 
The magnetizing current passes from the battery through the adjustable resistance 
and the galvanometer to a key, K, and thence, through the resistance to a 
two-way plug, P, which allows it to be sent either into the magnetizing coil of the 
ring A, which is the sample under test, or into the primary of a special induction coil, 
C, which is used for standardizing the ballistic galvanometer. A and C each have a 
secondary coil; the two secondaries are kept in series with one another, and their 
circuit is completed through the ballistic galvanometer Gg. A Kelvin balance, B, 
which is short-circuited when it is not required, serves to determine the constant of 
the current galvanometer G^. The key K is a rocker, working in mercury cups, which, 
when rocked to the left, connects a with e and 6 with f, and, when rocked to the 
right, connects a with c and h with d. In the latter position of the key, the full 
strength of the magnetizing current is in action, and hence, if the plug P is set so as 
to send the current into A, the sample is magnetized to a degree which represents one 
extremity of a magnetic cycle. Between the cups e and d of the rocking key are a 
short-circuit plug and also an adjustable resistance, Bg. When this short-circuit plug 
is inserted, the key K is simply a reversing key, and then, when rocked to the left, it 
reverses the magnetization of the ring, carrying it at once to the opposite end of the 
cycle. But when, instead of a short circuit between c and d, there is a resistance, the 
rocking of the key to the left applies to the sample a magnetizing current opposite in 
sign to the primitive current, but less than it in amount, and, consequently, carries 
its magnetism, not to the end of the cycle, but to a point which falls short of the end 
by an amount depending on the magnitude of the resistance Bg. The peculiarity of 
the method is that, by a single movement of the key, the magnetizing current is at 
6 K 2 
