I 
310 MR. WHEATSTONE ON NEW INSTRUMENTS AND PROCESSES FOR 
constant in its action, and convenient to manipulate with. It is quite unnecessary 
to use large elements in such investigations, for when considerable resistances are in- 
troduced in the circuits, which is most frequently the case, they produce no percep- 
tibly greater effect than smaller ones, and in all cases the measures may be as accu- 
rately determined by employing small elements as large ones. 
The voltaic element C consists of a glazed porcelain cell, two inches square and 
one inch and a half high, in the centre of which is placed a small porous cylinder of 
earthenware or wood, filled with a liquid amalgam of zinc, the space between the 
two cells being charged with a solution of sulphate of copper ; a slip of thin sheet 
copper bent round, and having one of its edges cut and turned over so that the wire 
of the circuit may be attached to it, or that it may dip into the amalgam of another 
similar cell, is placed in the solution. Fig. 3, Plate XVII. represents several such 
elements combined to form a series. It will be seen that, in principle, this is but a 
slight modification of Professor Daniell’s constant battery, liquid amalgam of zinc 
being employed, as in Mr. Kemp’s first experiment, instead of amalgamated zinc bars 
or plates, and the acid solution being dispensed with. This arrangement is, besides 
being very constant in its action, extremely economical and easy to manipulate. 
Any negative metal may be substituted for copper provided a solution of a salt of 
that metal be employed as the interposed liquid. 
§ 6 . 
The rheostat which I employ for circuits in which the resistance is comparatively 
small is represented at fig. 2. A. a is a cylinder of well-seasoned wood, on the surface 
of which a spiral groove is cut ; a thick copper wire is wound round the cylinder oc- 
cupying the groove, forming as it were the thread of a screw. Immediately above 
the cylinder and parallel with its axis is a triangular metal bar b, carrying a rider 
or slide c ; to this rider a spring d is attached, which constantly presses against the 
spiral wire, yielding to any slight inequality. One end of the spiral wire is attached 
to a brass ring e, against which a spring f presses, which is connected by means of 
a binding screw to one end of the circuit, the other end of the circuit is held by the 
binding screw which is in metallic connection with the triangular metal bar. On 
turning the handle h the cylinder is caused to move on its axis in either direction, 
and the rider c guided by the wire moves along the bar, advancing or receding ac- 
cording as the cylinder is moved to the right or to the left ; the rider coming in con- 
tact with a different point of the spiral wire, a different resistance is introduced into 
the circuit, consisting of that portion of the wire only which is included between 
the rider and the end of the wire connected with the spring f. The cylinder of 
the instrument I have constructed is 1Q1 inches in length, and inches in diameter ; 
the wire is of copper the 16th of an inch thick, and it makes 108 coils round the 
cylinder. The dimensions of the instrument, and the thickness, length, and material 
of the wire, may be varied according to the limits of the variable resistance required 
