270 
POPULAE SCIENCE EEVIEW. 
greater intensity in the electro-magnets. This mutual action 
soon builds up a very powerful battery. The other two bobbins 
meanwhile produce the current which is used externally. 
It is customary in other magneto-electric engines to excite 
the fixed electro-magnets once for all by means of a galvanic 
battery, the infinitesimal trace of residual magnetism sufficing 
to work the machine on subsequent occasions. Now a mere 
accident has led M. Gramme to dispense entirely with the use 
of the battery, for at the moment he was going to connect the 
electrodes of a few Daniell’s cells with the machine — which 
had been previously set in motion by an attendant — he found 
it evolving a strong current. Terrestrial magnetism had here 
anticipated him, and elicited in the soft iron the power he 
sought to induce. 
One of the great drawbacks in the machines of Wilde and 
Ladd is the heating of the armatures caused by the rapidity 
of their rotation, which varies from 1,000 to 2,400 revolutions 
per minute. This involves certain inconveniences, which have 
militated against their extensive adoption for practical purposes. 
The normal speed of the Oramme is only 300 revolutions per 
minute ; and it may be noticed that it is only when the 
external work is not proportional to the current generated that 
the temperature of the coils is slightly raised. By a proper 
management of the driving-power, iDut little if any heat may 
appear in the armatures, and a large fraction of the motive 
force may be converted into useful work. In no case, evidently, 
can the energy derived from a magneto-electric apparatus ex- 
ceed the power expended in producing it. 
There is another striking feature in this machine besides the 
continuity of its currents, viz., the possibility of increasing 
the number of electro-magnets without material inconvenience. 
The advantage that may be drawn from this is that several 
distinct currents may be derived from a single machine and 
applied to different purposes. 
The current produced is equal to that of a Bunsen battery 
of 525 cells, arranged in five rows. Its tension is therefore 
that of 1 05 cells, and its quantity that of five. The wire on 
the coils is No. 12, and weighs 165 lbs. — that on the electro- 
magnets is No. 11, and weighs 550 lbs. 
A fair knowledge of the vast energy of this machine may be 
derived from the experiments to which we referred in a pre- 
ceding paragraph. A platinum wire. No. 18 gauge and 15 ft. 
in length, was raised to a glowing heat, and the beautiful 
experiment illustrative of the specific resistance of platinum 
and silver was readily performed with 18 ft. of alternate pieces. 
Twenty-two and even 32 ft. of high conductivity copper wire, 96 
per cent, purity, was stretched between the terminals — the middle 
