in changing the dimensions of Iron and Steel bars. 93 
Examining the results in the two series of experiments, we 
see that where the current was passed, with all of the interposed 
resistance in the circuit, the scale went from 54°8 to 54°85, or 
moved ‘05 of a division in the first series of experiments; but 
in the second series the current was too feeble to effect a meas- 
ureable elongation, and it was not until } of the fine wire was 
out of the circuit that the scale-reading began to increase. In 
both series of experiments the rapid increase in the rate of 
elongation is noticeable after # of the fine wire was out of the 
circuit; the elongation, in both series of experiments, amounted 
to 18 divisions of the scale. The same amount of elongation 
always occurred when the ends of the copper wires were brought 
together, or when the circuit was as suddenly formed by plung- 
ing the wires into a cup containing mercury. Therefore, it is 
well established that a current of a definite strength will pro- 
uce the same amount of elongation whether that strength is 
suddenly or gradually attained. Indeed, in some of the experi- 
ments over three minutes were occupied in gradually decreasing 
the interposed resistance, until it was entirely out of the circuit, 
yet during this very slow increase of the current strength, the 
scale slowly and smoothly moved upward in its readings, and 
when all the interposed resistance had been passed over the 
elongation again equaled 1°3 division. 
he establishment of the above fact was of considerable im- 
a a é 
We will now observe. If around the rods we now passa voltaic 
current of graduall increasing strength, we will see the ring 
e anally displaced, and from the amount and direction of this 
placement, together with the knowledge of the wave-lengths 
of the rays of the sodium light, we can accurately determine the 
amount and direction of the motion of the ends of the rods. 
Ana) however, the current should have been panes, at once 
with its full intensity, there would have followed a sudden dis- 
