312 
MR. WHEATSTONE ON NEW INSTRUMENTS AND PROCESSES FOR 
meter are attended with considerable differences in the resistances of wires, it is more 
convenient to assume for the unit of resistance a wire of a given length and weight, 
which allows small differences to be very accurately determined. I shall in all my 
experiments, therefore, take for the unit of resistance a copper wire one foot in length, 
and weighing 100 grains. The diameter of this wire is the '0/1 of an inch, and it is 
intermediate to the numbers designated in commerce as fifteen and sixteen. 
§ 8. The Resistance Coils . 
It is frequently required to measure resistances much greater than can be effected 
by means of the rheostat, though the reduced length of its wire is considerable. I 
may wish to know, for instance, the resistance of the wire of the electro-magnets of 
my telegraphic apparatus, which is sometimes many hundred yards in length ; or 
that afforded by an extensive telegraphic line, or the resistance of a certain extent of 
an imperfectly conducting liquid. In all these cases and a variety of others I employ 
another instrument, which enables me to interpose in the circuit resistances to any 
amount, and yet to obtain by the conjoined use of the rheostat, which serves as its 
fine adjustment, any required degree of accuracy. This instrument is represented 
fig. l.D; it consists of six coils of fine silk-covered copper wire, about the awoth 
of an inch in diameter; two of these coils are fifty feet in length, the others are re- 
spectively 100, 200, 400, and 800 feet in length. The two ends of each coil are 
attached to short thick wires fixed to the upper faces of the cylinders, which serve 
to combine all the coils into one continued length ; the two wires a, h form the ex- 
tremities of the coils by which they are united to the circuit. On the upper face of 
each cylinder is a double brass spring moveable round a centre, so that its ends may 
rest at pleasure either on the ends of the thick connecting wires, or may be removed 
from them and rest only on the wood. In the latter position, the current of the cir- 
cuit must passthrough the coil, but in the former position, the current passes through 
the spring, and removes the entire resistance of the coil from the circuit. When all 
the springs rest on the wires, the resistance of the whole series of coils is removed, but 
by turning the springs so as to introduce different coils into the circuit, any multiple 
of 50 feet up to 1600 may be brought into it. 
As the measurement of these long lengths of wire cannot be accurately depended 
upon, it is advisable to ascertain the number of units of resistance in each coil, which, 
with the aid of the rheostat, may be easily effected. I find the resistance of the 
entire 1600 feet to be equivalent to 218,880 units of resistance, or feet of the standard 
wire. I occasionally employ an auxiliary series of coils combined in the same way as 
the preceding, consisting of six coils of the same wire, each 500 yards in length. 
The reduced length of this series is above 283 miles of the standard wire. By com- 
bining it with the preceding, I am able to measure resistances equal to 274^ miles. 
