1887.]  Haperiments on the magnetization of cron rods. 85 
iron wire put into the coils, the length and diameter of the cores 
being varied considerably. 
The magnetic induction was measured by the ballistic method. 
One turn of insulated wire was wrapped round the core at its 
middle and the ends connected with wires leading to a ballistic 
galvanometer placed at a distance of 9 metres. The value of the 
deflections was obtained by discharging a known quantity of 
electricity through the galvanometer: for this purpose we used a 
condenser of known capacity charged by Clark cells. In later ex- 
periments the value of the galvanometer deflections has been 
further checked by observing the deflection produced by suddenly 
turning a coil of known area through 180° in the earth’s magnetic 
field, sometimes round a vertical axis and sometimes round a 
horizontal one, this coil being in the galvanometer circuit. We 
have got with no special effort a magnetic induction of 28,000 
C.G.S. units when the magnetising force was about 1200, and are 
led to believe that considerably higher numbers might be reached 
if longer coils were used, and the ratio of the length ot the core to 
its diameter were increased. The highest induction yet recorded 
is 21,000 by Mr Shelford Bidwell, and Prof. Rowland gives 17,500 as 
the probable maximum attainable for iron. We have no reason to 
believe that with our present apparatus this is the limit we can 
obtain, but as we learn from Prof. Ewing that he is working at the 
same subject and has got inductions as high as 33,000, we have not 
pursued the matter further. 
For our main experiment however, the cores had to be cut 
across in the middle and separated by a small mterval. We found 
that the crevasse thus formed reduced the magnetization to a 
most surprising degree, and we will now proceed to describe our 
further experiments on this point. 
Experiments on the effects of cutting iron cores transversely to the 
lines of force. 
With the above described coils the induction was measured for 
a core of solid soft iron (Low Moor) of length 170 cms., about 5 ft. 
6 in., and diameter 3:2 cms., about 11in. This core was then cut 
into two equal parts, the ends being turned up in a lathe with no 
very special pains: the core was again put in the coils, as if to 
imitate the original uncut core. The induction for the cut bar was 
now measured and found to be more than 10 7 lower than for the 
uncut core. If the two parts instead of touching at the middle, 
were separated by 1 cm. the induction was now found to be reduced 
by 60 7. 
me have used smaller apparatus lately to inquire further into 
this point. 
The magnetizing coil was 16 cms. long and had 140 turns per 
