in changing the dimensions of Iron and Steel bars. 95 
first series, and regains the length it had before the current was 
passed around it. 
On passing the current with the whole of the fine wire in the 
circuit, we have, in the first series of experiments, an elongation 
of 05 of a division, but on making the circuit without the in- 
terposed fine wire, we have an elongation of 1°3 divisions; and 
if we now do not break the current, but gradually diminish its 
strength by increasing the interpolar resistance, we find that 
when the whole of the fine wire is again in the circuit that the 
elongation is yet -65 of a division : whereas when the circuit was 
at once formed with this same interposed resistance the rod was 
elongated only ‘05 of a division. 
The discovery of this most remarkable phenomenon was con- 
tained in the above experiments, but to be sure that my exper- 
iments should not mislead me, I repeated them several times, 
using every precaution to ensure their accuracy, and obtained 
results almost identical with those formerly observed. I am 
therefore confident that I have discovered a phenomenon worth 
of minute study, and I purpose to make it the subject of a 
Special investigation. 
Unfortunately, during the above experiments, I did not make 
a parallel series of determinations of the magnetic intensities of 
e rod during the successive stages of passing a current of in- 
creasing and of decreasing strength. Yet I can hardly believe 
that the magnetic intensity will be kept up with the persistent 
elongation of the rod when it is slowly demagnetized, but I 
think that it will be found that the magnetic intensity of the 
rod depends alone on the strength of the current traversing the 
helix. The phenomenon indeed shows that the molecules of 
the rod, on its elongation by magnetization, having been forced 
=~ new positions, that either by what might be well called a 
ugnetic set”* or from molecular friction, the molecules 
retained these new positions with such persistence, that it 
required the sudden shock of the induced current, produced on 
breaking the circuit, to cause them to rush to their positions of 
stable equilibrium. 
Effects observed on making and breaking separate currents in the 
component helices of the compound helix. 
uae these experiments two batteries were used. In the outer 
ix I made and broke a current from 16 cells, arranged four 
raed and four in series. Jn connection with the inner helix, 
used a battery of 25 cells, connected five in a row and five in 
Series. The experiments are interesting as showing the effects 
*The term : . 
name magnetic set,” as appplied above, is from analogy an appropriate 
Pt the phenomenon; but it cannot well be so applied, for Dr. Joule has 
ted “* i i e residual magnetism an iron 
Y appropria ic set” th 
Tod retains after ite sagen. ot TFG tatty 
