When a current is sent through the voltameter, iodine separates out at the platinum anode, but, on account of its great density, remains at the bottom of the glass vessel. The current density at the cathode should not exceed 1 ampère per 200 sq. cm. surface, otherwise insoluble periodate is liable to be formed. After the electrolysis is stopped, the zinc cathode is at once removed, and the solution stirred up; it will have a reddishbrown colour, due to the liberated iodine. The amount of iodine liberated is 0'001314 gramme per coulomb, and in order to determine the amount liberated by the electrolysis, the brown solution is titrated with a standard solution of pure sodium thiosulphate, a convenient strength being found to be 12.845 grammes of pure sodium thiosulphate to 1000 cub. cm. water; I cub. cm. of this solution being equivalent to o'00657 gramme iodine or 5 coulombs of electricity per cubic centimetre of thiosulphate required in the titration. The vessel containing the brown solution should be placed on a sheet of white paper, and a burette filled with the sodium thiosulphate solution fixed in a retort stand, so as to be able to run it into the iodine solution. The thiosulphate is run in slowly, and, when the brown solution is nearly decolorized, drop by drop. When the last trace of colour vanishes the burette is closed, and the amount of liquid that has been run out noted. Multiplying the number of cubic centimetres of liquid used by five, and dividing by the time of the electrolysis in seconds, the current in ampères is obtained. If greater accuracy is desired the burette should be weighed before and after the experiment, and the volume of thiosulphate solution used calculated from its specific gravity and the weight used. Also, when the titration has been nearly completed, a little clear starch solution may be added to the iodine solution, and the exact point when the titration has been completed judged by the vanishing of the blue coloration produced by the free iodine on the starch. 170. When very small currents are to be measured, such as may be employed in the calibration of high resistance sensitive M galvanometers, the time of electrolysis must be considerable, and the form of the voltameter must be altered to prevent the diffusion of the iodine through the solution, the most convenient form being a U-tube with an asbestos plug at the bend in the tube. If the thiosulphate solution has been standing for any length of time it is liable to decompose, and should be tested with a standard iodine solution. 171. The following experiment will illustrate the accuracy of the above method of measuring current. A current from three secondary cells was sent through an iodine voltameter in series with a 50-ohm standard coil and an adjustable resistance. The current was regulated by the adjustable resistance, so that the potential difference at the terminals of the 50-ohm coil just balanced the E.M.F. of a Clark standard cell of 1'434 volts. The current was allowed to flow for 33 min. 20 sec. The zinc was then removed, and the solution stirred up, and titrated with the standard solution of sodium thiosulphate (5 coulombs per cubic centimetre). It was found to require 11°47 cub. cm. Current as determined by Clark cell = Current as determined by voltameter = I'434 =0'0286 amp. 50 11'47 × 5 = 0·0286amp. 2000 LORD KELVIN'S CURRENT BALANCES. 172. We have already described the principle of the absolute current balance, and from the nature of the calculation for the current passing through it, it will be seen that its indications are independent of the magnetic force of the earth, the controlling moment being due to a weight in the scale-pan of the balance. This principle has been employed by Lord Kelvin in a series of instruments for the measurement of currents, the instruments being called current balances. These instruments, however, differ from the absolute current balance in this respect: that whilst in the former the current is calculated from the dimensions of the apparatus and the balancing weight employed; in the latter, the constant of the instrument is determined by the aid of a copper voltameter. On account of the invariability of this constant, the balances may be employed as secondary standard current measuring instruments. In each of the balance instruments, except the kilo-ampère balance, each movable ring is actuated by two fixed rings— all three approximately horizontal (see Fig. 86). There are two such groups of three rings-two movable rings attached to the two ends of a horizontal balance arm pulled, one of them up and the other down, by a pair of fixed rings in its neighbourhood. The current is in opposite directions through the two movable rings to practically annul disturbance due to horizontal components of terrestrial or local magnetic forces. In all the instruments the balance arm is supported by two trunnions, each hung by an elastic ligament of fine wire, through which the current passes into and out of the circuit of the movable rings. In all the balance instruments in which the movable ring is between the two fixed rings, the mid-range position of each movable ring is in the horizontal plane nearly midway between the two fixed rings which act on it. The current goes in opposite directions through the two fixed rings, so that the movable ring is attracted by one and repelled by the other. The position of the movable ring equidistant from the two fixed rings, is a position of minimum force, and the sighted position, for the sake of stability, is above it at one end of the beam and below it at the other, in each case being nearer to the repelling than to the attracting ring by such an amount as to give about o'2 per cent. more than the minimum force. The balancing is performed by means of a weight which slides on an approximately horizontal graduated arm attached to the balance; and there is a trough fixed on the right-hand end of the balance, into which a proper counterpoise weight is placed, according to the particular one of the sliding weights in use at any time. For fine adjustment of the zero, a small metal flag is provided, as in an ordinary chemical balance. This flag is actuated by a fork, having a handle below the case outside. To set the zero, the left-hand weight is placed with its pointer at the zero of the scale, and the flag is turned to one side or the other until it is found that, with no current in the rings, the balance rests in its sighted position. To measure a current, the weight is slipped along the scale until the balance rests in its sighted position. The strength of the current is then read off approximately on the fixed scale (called the inspectional scale) with the aid of the finely divided scale for more minute accuracy, according to the explanations given below. Each number on the inspectional scale is twice the square root of the corresponding number on the fine scale of equal divisions. The slipping of the weight into its proper position is performed by means of a self-releasing pendant, hanging from a hook carried by a sliding platform, which is pulled in two directions by two silk threads passing through holes to the outside of the glass case. Four pairs of weights, sliding and counterpoise, of which the sledge and its counterpoise constitute the first pair, are supplied with each instrument. These weights are adjusted in the ratios of 1 : 4: 16: 64, so that each pair gives a round number of ampères, half-ampères, quarter-ampères, or of decimal sub-divisions or multiples of these magnitudes of current, on the inspectional scale. The useful range of each instrument is from 1 to 100 of the smallest current for which its sensibility suffices, these ranges in the centi-ampère, deci-ampère, and deka-ampère balances being from I to 100 centi-ampères, deci-ampères, and ampères, respectively. The balances are designed to carry 75 per cent. of their maximum current continuously, and their maximum current long enough for all standard purposes. The following table gives for each type of instrument the value per division of the inspectional scale corresponding to each of the four pairs of weights :— |