DETERMINING THE CONSTANTS OF A VOLTAIC CIRCUIT. 
307 
whatever of an electric current ; when speaking- of a single element I shall term it a 
rheomotive element, and what is usually called a voltaic or thermo-electric pile or 
battery I shall term a rheomotive series. I shall still use the ordinary expressions 
when I have to refer to the specific sources of the production of electric currents, but 
when I employ the general terms they must be understood to apply to all these 
sources indifferently. 
The want of a general term to designate an instrument to measure the force of an 
electric current without reference to its particular construction has been long felt. I 
shall use the word Rheometer for this purpose, continuing occasionally to employ 
galvanometer, voltameter, & c. to distinguish the particular instruments to which these 
names have been applied, though perhaps the terms Magnetic, Chemical, Calorific, 
&c. Rheometer would be more appropriate. 
This may be the proper place to explain a few other terms which I have frequent 
occasion to use, though not in the course of the present communication. By Rheo- 
tome is meant an instrument which periodically interrupts a current, and by Rheo- 
trope an instrument which alternately inverts it. A Rheoscope is an instrument for 
ascertaining merely the existence of an electric current. The word Rheostat will be 
hereafter explained. 
I have not introduced these terms, which will be found greatly convenient and will 
enable us to state general propositions much more clearly, without good authority. 
The word Rbeophore was employed by Ampere to designate the connecting wire of 
a voltaic apparatus, as being the carrier or transmitter of the current ; and the word 
Rheometer, first proposed by Peclet as a synonym for galvanometer, has been gene- 
rally adopted by the French writers on physics. 
§ 4 . 
The method of obtaining the constants of a rheophoric circuit adopted by Fechner, 
Lenz, Pouillet, &c., in their experimental verifications of Ohm’s theory, is essentially 
the following : — 
The resistance of a circuit is determined by observing the force of the current, first 
without any extra interposed resistance in the circuit, and afterwards when a known 
resistance is added. Then 
and F' = 
E 
R + r ‘ * F' 
R + r 
"It - ’ 
from which equation the value of R, all the others being known quantities, is easily 
F . . 
deduced. R = | ;^r. The electro-motive force of a circuit is ascertained by mul- 
E 
tiplying the force of the current into the total resistance : tor since F = E = F R. 
The principle of this method is extremely simple, but the difficulty of determining 
immediately the force of a current by means of a galvanometer is an obstacle to its 
2 s 
MDCCCXLIII. 
