DETERMINING THE CONSTANTS OF A VOLTAIC CIRCUIT. 
311 
to be introduced into the circuit, and the degree of accuracy with which these 
changes are required to be measured. 
Fig. 2 represents the arrangement of a thermo-electric circuit in which this in- 
strument is interposed. C is the thermo-electric element ; B the galvanometer, which 
in this case must not have numerous coils of fine wire as in the preceding arrange- 
ment, for this would introduce too great a resistance into the circuit, but must con- 
sist of a single thick plate or wire making a single convolution ; or, which I think is 
preferable, the method of diverting a portion of the current from the wire of a deli- 
cate galvanometer described in § 15. may be adopted. Any rheomotor in which the 
resistance is small may be employed in conjunction with this form of the rheostat, 
instead of a thermo-electric element, as represented. 
The rheostat, especially under the form last described, may be usefully employed 
as a regulator of a voltaic current in order to maintain for any required length of 
time precisely the same degree of force, or to change it in any desired proportion. 
Interposed in the circuit of an electro-magnetic engine, however the rheomotor may 
vary in its energy, the same velocity may be constantly restored by turning the 
cylinder of the regulator to the left or to the right, according as the velocity increases 
or decreases ; or any different velocity, within given limits, may be obtained by ad- 
justing the rheostat accordingly. Since the consumption of materials in a voltaic 
battery in which there is no local action decreases in the same proportion as the in- 
crease of the resistance in the circuit, this method of altering the velocity has an ad- 
vantage which no other possesses, the effective force is always strictly proportional to 
the quantity of materials consumed in producing the power, a point which, if further 
improvements should ever render the electro-magnetic engine an available source of 
mechanical power, will be of considerable importance. 
In volta-typing operations the advantage of using the rheostat is obvious. By 
varying the rheostat from time to time so as to keep the needle of a galvanometer to 
the same point, a current of any required degree of energy maybe maintained, with- 
out any notable increase or diminution, for any length of time ; and, as the nature of 
the deposit, when the solution from which it is made remains the same, varies only 
with the force of the current and the magnitude of the surface on which the metal 
is reduced, when once a good effect has been obtained the same circumstances may 
be reproduced with ease and certainty, and the effects of chance entirely eliminated. 
In the operations of voltatyping, electro-gilding', &c., and in the production of 
Nobili’s colours, the advantage of using the rheostat is obvious. 
This however is not the place to dilate on this subject. 
§ 7. Standard of Resistance. 
It is of the highest importance to have a correct standard of resistance, and one 
that can easily be reproduced for the purpose of comparison. A copper wire of a 
given length and diameter might be employed, but as very small differences of dia- 
