Introduction to physical chemistry / by James Walker.

  • Walker, James, Sir, 1863-1935.
Date:
1903
Catalogue details

Licence: In copyright

Credit: Introduction to physical chemistry / by James Walker. Source: Wellcome Collection.

    241/388 (page 225)
    W we place a known resistance in the box R, and move the contact C along the platinum wire until the galvanometer shows no deflection. We know then that R:W ^ a :b. But the platinum wire is of uniform resistance, so that the resistance of a is to the resistance of b as the length of a is to the length of b, and these lengths may be read off directly on the scale along which the wire is stretched. When the resistance to be measured is that of an electrolyte, it is generally impossible to use a galvanometer owing to the polarisa- tion at the electrodes when a steady current flows through the electrolyte. This polarisation may be avoided by the use of an alternating current instead of a direct current, but then the galvanometer is use- less to indicate the alternating current. Its place, however, may be taken, as Kohlrausch has shown, by the telephone, which is silent when no current passes through it, but sounds when it is traversed by alternating currents. The alternating current is got from the secondary coil of a small inductorium, worked by the battery B, which is usually a single bichromate cell. The modified arrangement is shown in Fig. 40, where S is the electrolytic solution, T the telephone, and I the induction coil. The vessel which contains the solution is generally of the type proposed by Arrhenius, and shown in Fig. 41 in natural size. The electrodes are made of stout platinum discs, fitting closely to the cylindrical vessel. A short platinum stem from each is sealed into a glass tube, held firmly in the ebonite cover C. The connecting wires are passed down the glass tubes till they make contact with the platinum wires by means of a drop of mercury introduced into each tube. This form is most suitable for very small conductivities. For solutions which have greater conductivity, a modified type with a narrower end may be employed, which has the advantage of being considerably cheaper on account of the much smaller size of the platinum electrodes used in its construction. If the platinum electrodes are bright, there is no sharp minimum of sound in the telephone at any position of the sliding contact. It is therefore necessary to cover the surface of the platinum with a coating of finely-divided platinum, Q