ELECTRODYNAMIC QUALITIES OF METALS. 
05 
with the top of the barrel, produced by applying a transverse outward pressure of 
(approximately) 1000 lbs. per square inch to the iron of the barrel. The ordinates 
are given in divisions of the magnetometer scale, and the abscissas in divisions of the 
scale of the battery galvanometer. It is interesting to note that the critical value 
of the magnetism is for this position of the magnetometer nearly 80 times the vertical 
component of the earth’s magnetic force at Glasgow. 
216. By placing the magnetometer at lower levels relatively to the barrel, it was 
found that the values of the current required for maximum and for zero effect, by 
“ on ” and “ off,” were less the nearer the level of the magnetometer needle was to the 
middle of the length of the bar. The following table shows the principal effects for 
— M at four different levels of the magnetometer needle :— 
Distance* of top 
of barrel above 
level of magneto¬ 
meter needle. 
Maximum deflec¬ 
tion before 
critical value was 
reached. 
Magnetizing 
current 
for maximum 
deflection. 
Critical value 
of magnetizing 
current. 
Centims. 
0 
32-8 
110 
430 
10-5 
31 
130 
345 
21-0 
14 
100 
265 
31-5 
3 
100 
147 
The whole of the results for the — M in the last three cases are given in curves (2) 
of Diagram YU. 
217. The magnetometer needle was placed on a level with the top of the barrel, and 
at a distance of two metres from its axis. Experiments were then made to find the 
total magnetization produced by different strengths of magnetizing current, and the 
effects on it of ten successive applications and removals, wdiile the current was still 
flowing, of a hydrostatic pressure of 1000 lbs. per square inch; also to find the 
total residual magnetism after the removal of the magnetizing current, when the tube 
was left solely under the influence of the earth’s vertical magnetic force, and the effect 
on it of ten successive applications • and removals of the same hydrostatic pressure, 
while the bar was still under this magnetizing influence. The general character of the 
results of these experiments is difficult to describe in words, but can be seen by 
inspecting Diagram VIII., in which the curves marked A show the total magnetiza¬ 
tion, those marked B the residual magnetization. The abscissas of these curves are 
proportional to the magnetizing forces and the ordinates to the observed magnetization: 
but they have inadvertently been so drawn that their negative ordinates show magneti¬ 
zation of the same polarity to that produced by the inductive influence of the earth’s 
vertical force, and their positive ordinates magnetization of the opposite polarity. The 
full lines in each set of curves show the magnetization before, the dotted lines after, ten 
“ ons ” a “ offs ” of and pressure of 1000 lbs. per square inch. 
* The length, of the barrel and of the magnetizing coil was about 90 centims, 
MDCCCLXXIX, K 
