DETERMINING THE CONSTANTS OF A VOLTAIC CIRCUIT. 
327 
binding 1 screws C, Z, and the ends of the galvanometer wire being attached to the 
screws a and b ; in the position of the instrument represented in the figure, the 
springs G and H resting respectively on the insulated pieces of brass D and N, the 
principal portion of the current passes through the resistance coil L, and the current 
is afterwards equally divided between the coil of the galvanometer and the resistance 
coil K. But when the circle is moved in the direction of the arrow, the springs G, H 
leave the brass plates, and rest on the wood, while the spring I is brought into contact 
with the plate E ; both of the resistance coils are now thrown out of the circuit, and 
the current passes wholly through the wire of the galvanometer. 
It is almost unnecessary to state that this instrument can only be used in con- 
junction with the galvanometer to which its resistance coils K and L have been 
adjusted. 
In some cases, when an experiment has been performed with a current of a certain 
degree of intensity, it is required to repeat it with currents of other degrees of 
strength, the proportions of which to the first current shall have been accurately de- 
termined. The instrument above described readily affords the means of doing this. 
It may thus be ascertained whether the electro-motive force in any particular com- 
bination varies or remains constant when the energy of the current changes. 
^ 19. Process to determine the Degrees of Deviation of the Needle of a Galvanometer 
corresponding to the Degrees of Force ; and the Converse. 
When the electro-motive force in the circuit remains constant, the force of the 
current is simply proportional to the resistance or reduced length of the circuit. If 
therefore the total resistance of the circuit, when the needle stands at 1 °, be deter- 
mined, and if then, by means of the rheostat and resistance coils, the resistance be 
successively reduced to 3 -, & c., the corresponding forces of the current will 
be 2 , 3, 4, 5, &c. Conversely, if the reduced lengths a, b, c, d, &c. necessary to be re- 
moved from the circuit in order to advance the needle from each degree to the one 
next above it be successively ascertained, the forces corresponding to these successive 
degrees will be 
l l 1 l 
R’ R-ffl’ R -(« + *)’ R-{a + b + c)’ &C * 
By the above processes, the relations between the degrees of force and those of the 
galvanometric scale may be far more readily determined than by either of the inge- 
nious methods of Nobili, Becquerel or Melloni. When we consider the changes 
to which the needle of a delicate galvanometer, especially if it be astatic, is subject 
from the influence of strong currents, the vicinity of magnets, and, in a less degree, 
from changes of temperature, and in the intensity of the earth’s magnetism, the im- 
portance of having an easy means of re-graduating the instrument, and of detecting 
the changes it has undergone, will not be esteemed too lightly. 
