DETERMINING THE CONSTANTS OF A VOLTAIC CIRCUIT. 
309 
§ 5 . 
The first instrument is represented in Plate XVI. fig - . 1. A. g is a cylinder of wood, 
and A is a cylinder of brass, both of the same diameter, and having - their axes parallel 
to each other. On the wood cylinder a spiral groove is cut, and at one of its extre- 
mities a brass ring is fixed, to which is attached one of the ends of a long wire of 
very small diameter, which when coiled round the wood cylinder fills the entire 
groove, and is fixed at its other end to the remote extremity of the brass cylinder. 
Two springs, j and k, pressing one against the brass ring on the wood cylinder, and 
the other against the extremity of the brass cylinder h, are connected with two 
binding screws for the purpose of receiving the wires of the circuit. The moveable 
handle m is for turning the cylinders on their axes. When it is placed on the 
cylinder h and is turned to the right, the wire is uncoiled from the wood cylinder and 
coiled on the brass cylinder, but when it is applied to the cylinder y and is turned 
to the left, the reverse is effected. The coils on the wood cylinder being insulated 
and kept separate from each other by the groove, the current passes through the 
entire length of wire coiled upon that cylinder, but the coils on the brass cylinder 
not being insulated the current passes immediately from the point of the wire which 
is in contact with the cylinder to the spring k. The effective part of the length of 
the wire is therefore the variable portion which is on the wood cylinder. 
In the instrument I usually employ the cylinders are six inches in length and 
inch diameter, the threads of the screw are forty to the inch, and the wire is of 
brass the t^o^ 1 °f an inch in diameter. I employ a very thin wire and a badly con- 
ducting metal, in order that I may introduce a greater resistance into the circuit. 
A scale is placed to measure the number of coils unwound ; and the fractions of a 
coil are determined by an index which is fixed to the axis of one of the cylinders and 
points to the divisions of a graduated circle. 
As the principal use of this instrument is to adjust or regulate the circuit so that 
any constant degree of force may be obtained, I have called it a Rheostat. 
Plate XVI. fig. 1 shows the arrangement of the circuit when prepared for an ex- 
periment. B is a delicate galvanometer with an astatic needle furnished with a mi- 
croscope for reading off the divisions of the circle, which greatly facilitates the ob- 
servations. C is the rheomotor. 
I must here digress for a moment to describe the voltaic element which I have 
employed in most of my rheometric researches, and which I have found to be very 
municated to the author by Mr. Wheatstone. See the Annals, vol. lii. p. 526.” I have referred to this 
volume and find it was published in the latter part of 1841, wdiile my communication to Professor Jacobi was, 
as above stated, made in August 1840. I may also mention, that the experimental process employed by Pro- 
fessor Poggendorff had no resemblance whatever to mine, and the result he sought was likewise different ; 
the mathematical principle of the method was however in the single case he investigated undoubtedly the 
same. 
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