of Edinburgh , Session 1870 - 71 . 493 
resistance is felt. The moment that there is hard work to be done 
in the external circuit, the strength of the arm is put to the proof. 
When water is decomposed by the machine, the strain upon the 
arm does not rise beyond a certain amount, at whatever speed the 
handle be driven. In working an induction coil, the load on the 
arm appears capable of rising to any extent, and the length or 
density of the spark bears something like a proportion to the 
burden of work. With an electric resistance great enough, and an 
inexhaustible driving power, there seems no limit to the electric 
effect attainable, and that, too, with little increase of speed. 
When a tangent galvanometer is interposed in the external 
circuit, something may be learned of the way this takes place. 
With an easy circuit, where little difficulty is felt in driving, a 
current of about 60° may be got. When a thin wire is now inter¬ 
posed, the needle does not reach this point, for the wire (iron wire 
gL inch in diameter) melts or ignites between 30° and 40°, and yet 
while the heating lasts the strain is enormously greater than before. 
If the galvanometer be inclosed in the internal circuit, and the 
wire melted in the electric circuit, just at the point when the heat¬ 
ing begins, the needle takes a sudden swing upwards. Thus, if it 
be at 20° before the heating sets in, it will rise to 30°, and stay 
there till the wire melts, when, if the motion be continued, it again 
takes a start upwards. If the magnetic coil be detached from the 
coil of the electro-magnet, and if its function be performed by one 
Bunsen cell, this increase of load is not felt, a greater effect in 
the external circuit being only attainable by an increase in velocity, 
and the same holds with a battery of permanent magnets. 
That two separate coils, by being imbedded in the same piece of 
iron, should thus act upon each other seems strange. One might al¬ 
most think that it arose from the particles of iron refusing to polarise 
and unpolarise quick enough. The maximum speed of revolution 
of the armature is about 2500 times a minute. The driving gear 
multiplies 22 times, so that this speed is nearly as much as the 
arm can effect. A particle of iron would have thus 10,000 times 
to polarise and unpolarise in a minute. A little consideration will 
show, however, that it is from no such incapacity on the part of 
the iron; for at the same rate of revolution, the two effects are felt 
with the different circuits. Speed in these cases, therefore, has not 
3 u 
VOL. VII. 
