DR. J. HOPKINSON ON THE MAGNETISATION OF IRON. 
457 
are faced true and square ; a space is left between the magnetising coils BB for the 
exploring coil D, which is wound upon an ivory bobbin, through the eye of which one 
of the rods to be tested passes. The coil D is connected to the ballistic galvanometer, 
and is pulled upwards by an india-rubber spring, so that when the rod C is suddenly 
pulled back it leaps entirely out of the field. Each of the magnetising coils B is 
wound with twelve layers of wire, 1T3 mm. diameter, the first four layers being 
separate from the outer eight, the two outer sets of eight layers are coupled parallel, 
and the two inner sets of four layers are in series with these and with each other. 
The magnetising current therefore divides between the outer and less efficient convo¬ 
lutions, but joins again to pass through the convolutions of smaller diameter. The 
effective number of convolutions in the two spools together is 2008. Referring to 
fig. 1, the magnetising current is generated by a battery of eight Grove cells E, its 
value is adjusted by a liquid rheostat F, it then passes through a reverser G, and 
through a contact breaker H, where the circuit can be broken either before or at the 
same instant as the bar C is withdrawn ; from H the current passes round the 
magnetising coils, and thence back through the reverser to the galvanometer K. The 
galvanometer K was one of those supplied by Sir W. Thomson for electric light work, 
and known as the graded galvanometer, but it was fitted with a special coil to suit the 
work in hand. The exploring coil D was connected through a suitable key with the 
ballistic galvanometer L. Additional resistances M could be introduced into the 
circuit at pleasure, and also a shunt resistance N. With this arrangement it was 
possible to submit the sample to any series of magnetising forces, and at the end 
of the series to measure its magnetic state; for example, the current could be passed 
in the positive direction in the coils B, and gradually increased to a known maximum ; 
it could then be gradually diminished by the rheostat F to a known positive value, 
or it could be reduced to zero ; or, further, it could be reduced to zero, reversed by the 
reverser G, and then increased to any known negative value. At the end of the 
series of changes of magnetising current, the circuit is broken at H (unless the 
current was zero at the end of the series), and the bar C is simultaneously pulled 
outwards. Three successive elongations of the galvanometer L are observed. From 
the readings of the galvanometer K, the known number of convolutions of the coils 
B, and an assumed length for the sample bars, the intensity of the magnetising 
force $ is calculated. The exploring coil D had 350 convolutions. From its 
resistance, together with that of the galvanometer with shunts, the sensibility of the 
galvanometer, its time of oscillation, and its logarithmic decrement, a constant is 
calculated which gives the intensity of induction in the iron from the mean observed 
elongation of the galvanometer. The resistances have been corrected in the 
calculation for the error of the B.A. unit, and both galvanometers were standardised 
on the assumption that a certain Clark’s cell had an electromotive force of 
1'434X 10 8 C.G.S. units. This Clark’s cell had been compared to and found identical 
with those tested by Lord Rayleigh. 
