326 
MR. WHEATSTONE ON NEW INSTRUMENTS AND PROCESSES FOR 
§ 18 . 
It would greatly facilitate our quantitative investigations if we had a certain and 
ready means of ascertaining what degree of the galvanometric scale indicated half 
the intensity corresponding to any other given degree. The properties of diverted 
currents, established by the theory of Ohm, and fully confirmed by experiment, enables 
me to propose a simple method by which this object may be completely attained. 
If a wire of the same length, thickness and conductibility as that of the galvano- 
meter be placed so as to divert a portion of the current from it, it is obvious that one- 
half of the current will pass through the galvanometer wire, and the other half 
through the diverting path. Though it simplifies the consideration to suppose the 
extra wire to have the same length, diameter and conducting power, it is easy to see 
that the same result follows if the two wires present the same resistance which they 
do whenever s' c' l = s c If the added wire produced no alteration in the intensity 
of the principal current, one-half of the former force would act upon the galvano- 
meter ; but this is not the case, the addition of the wire produces the same effect as 
doubling the section of the galvanometer wire would do, and the total resistance of 
the circuit is therefore diminished. If the strength of the original current when it 
E 
passes wholly through the galvanometer = ^ — r (r being the resistance of the galva- 
nometer wire, and R all the other resistances in the circuit)? 
will be the strength 
of the principal current when the extra wire is added ; if now an additional resistance 
= that is to say, a wire whose resistance is equal to half that of the galvanometer 
wire, be added to the principal portion of the circuit, the intensity will be again 
E 
— , and the force acting on the galvanometer will be exactly half what it 
~2 
was at first. 
The construction and use of the instrument (fig. 8) will now be easily understood. 
A is a square piece of wood, having two insulated pieces of brass, D, N, inlaid on its 
surface, on which are fixed the binding screws C, Z and a; B is a circle also of wood, 
moveable round its centre ; upon this moveable circle are fixed the insulated piece of 
brass F, with the binding screw b upon it, and three springs G, H, I, the free ends of 
which press on the board A. A coil of wire K, the equivalent resistance to the wire 
of the galvanometer, measured by the process described in § 16, i? connected by its 
two ends with the brass plate F and the spring G ; and another ceil, L, the resistance 
of which is one-half that of the former, is similarly interposed between the brass plate 
and the spring H. A short wire immediately connects the plate F with the spring I. 
E is a nut or pin by which the moveable circle is moved through a small arc. 
The wires proceeding from the poles of a rheomotor being connected with the 
