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The readings of the ends of the pointer, marked A and B, are taken as positive when the deflections are counter-clockwise as measured from the zero divisions of the scale.

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* If the tube is not divided into cubic centimetres, the value of the divisions

ought to be ascertained by experiment.

Similarly for the observations during the second half of the experiment.

The results are tabulated as follows:

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Constant of Galvanometer by calculation

Time of oscillation at place of observation

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SECTION LVI.

STANDARDISING CURRENT METERS BY THE

COPPER VOLTAMETER.

Apparatus required: Current-measuring instrument to be standardised (an instrument intended to measure accurately one or two ampères is suitable); storage cells, copper depositing cell, copper electrodes, plug-key, resistance coils, accurate balance.

In the previous exercise the amount of electrolysis produced by the passage of a current was estimated by the volume of gas generated. Since volumes cannot be directly determined to the same degree of accuracy as masses can be weighed, more accurate results are obtained by the use of an electrolyte of which one of the products of decomposition is a solid which can be weighed. Silver and copper salts are found to be suitable for the purpose, as secondary reactions may be more completely avoided than in the case of other metals. For the most accurate work the silver voltameter is used*, but its manipulation requiring great care, and the materials being expensive, copper may be substituted, and with proper precautions an accuracy of one part in a thousand may be attained.

In the present exercise a solution of copper sulphate in water will be used, and a current meter will be standardised by the weight of copper deposited. The example is worked out on the supposition that the instrument to be standardised is a Kelvin Current Meter, but any other accurate instrument such as that shewn in Fig. 106 may be substituted.

Prepare a 20% solution of copper sulphate, by adding 120 grams of copper sulphate crystals to 480 grams of water. When the crystals have dissolved, filter into the beaker provided and add one or two c.c. of strong sulphuric acid.

* Lord Rayleigh and Mrs Sidgwick "On the electrochemical equivalent of silver and on the absolute electromotive force of Clark cells," Phil. Trans. 1884.

Clean the three copper plates provided, two of which are to serve as anodes and the third as cathode, with sandpaper. Dip the one which is to serve as cathode into dilute nitric acid for about three minutes, then remove it and place the three in dilute sulphuric acid for about three minutes, finally wash under a stream of tap-water.

Place the first two plates in the side clips A of the stand (Fig. 106) provided, and the third plate in the centre clip.

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Join up the copper voltameter, a plug-key K and an adjustable resistance to the instrument to be standardised, through two storage cells or other battery of small resistance giving an electromotive force of about 4 volts. Be careful to connect the terminals of the battery in such a way that the current passes through the copper sulphate solution from the outside plates (anodes) to the central plate (cathode) of the voltameter. If the instrument to be standardised gives deflections in different directions when the current is reversed, a reversing key should be inserted so that the directions of the current can be changed rapidly in the current meter without being reversed in the voltameter; and that change should be made in the middle of the experiment.

Adjust the resistances until the current is of convenient amount to be measured, then take out the plug key K.

Raise the plates out of the solution, take the copper cathode from its clip, wash it in clean water, and dry first in a sheet of filter-paper, then before a fire, heating coil, or gas flame, taking care not to heat the plate appreciably. When the plate is dry and at the temperature of the room weigh it to 1 milligram.

As the absolute weight is required, the plate must be weighed on both sides of the balance according to the method described on p. 45, and trustworthy weights only must be used. Read the zero of the instrument.

Replace the cathode and lower the copper plates into the solution, make the circuit at K at a time to be noted in minutes and seconds, and as soon as possible take a reading of the instrument to be standardised.

Take readings every five minutes for forty-five minutes or an hour.

The true readings will be the differences between the actual readings and the mean of the zeros read at the beginning and end of the experiment.

Note the temperature of the electrolyte.

While the electrolysis is going on, clean a beaker, fill it with clean water, and add a few drops of sulphuric acid.

At the end of the above period break the circuit at K, noting the time accurately, and read the zero of the instrument.

Raise the electrodes, remove the cathode and dip it as quickly as possible into the prepared acidulated water, then hold it under a gentle stream of water from the tap for a minute. Now dry it as before, first in a pad of clean filterpaper, then before a fire, heating coil or flame, and after cooling weigh again to 1 milligram.

The relation between the current C, the electrochemical equivalent of copper z, and the weight W deposited in a given time t is

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In the absence of all secondary reactions the value of z should be constant. But owing probably to the presence of dissolved oxygen, the amount of copper deposited is not strictly proportional to the current and depends to a small extent on the temperature. These effects can be taken into account by making z depend on the current density at the cathode and on the temperature. Thomas Gray, who has carefully investigated the amounts of copper deposited under different conditions, has

S. P.

18

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