On the Adjustment of a Reflecting Galvanometer.

In adjusting a reflecting galvanometer, we have first to place it so that the magnet and mirror may swing quite freely. This can be attained by the adjustment of the levelling screws on which the instrument rests. There is generally a small aperture left in the centre of the coils opposite to that through which the light is admitted to the mirror. This is closed by a short cylinder of brass or copper which can be withdrawn, and by looking in from behind, it is easy to see if the mirror hangs in the centre of the coils as it should do.

The lamp and scale are now placed in front of the mirror, the plane of the scale being approximately parallel to the coils, and the slit through which the light comes rather below the level of the mirror.

The magnet and mirror are adjusted, by the aid of the control magnet, until the light is reflected towards the scale. The position of the reflected beam can easily be found by holding a sheet of paper close to the mirror so as to receive it, moving the paper about without intercepting the incident beam. By moving the control magnet, and raising or lowering the scale as may be required, the spot may be made to fall on the scale.

The distance between the galvanometer and scale must now be varied until the image formed on the scale is as clear and distinct as possible; and, finally, the control magnet must be adjusted to bring the spot to the central part of the scale, and to give the required degree of sensitiveness.

As we have seen, the sensitiveness will largely depend on the position of the control magnet. Its magnetic moment should be such that when it is at the top of the bar which supports it, as far, that is, as is possible from the needle, the field which it alone would produce at the needle should be rather weaker than that due to the earth. If this

be the case, and the magnet be so directed that its field is opposite to that of the earth, the sensitiveness is increased at first by bringing the control magnet down nearer to the coils, becoming infinite for the position in which the effect of the control magnet just balances that of the earth, and then as the control magnet is still further lowered the sensitiveness is gradually decreased.

The deflexion observed when a reflecting galvanometer is being used is in most cases small, so that the value of measured in circular measure will be a small fraction; and if this fraction be so small that we may neglect 3, we may put sin == tan p (see p. 45) and we get i = kp.

With a sensitive galvanometer in which the coils are close to the magnet the ratio of the length of the magnet to the diameter of the coil is considerable, and the galvanometer constant is a function of the deflexion; so that kis not constant for all deflexions in such an instrument, but depends on the angle . If, however, the deflexions employed be small we may without serious error use the formula ik, and regard k as a constant.

72. Determination of the Reduction Factor of a
Galvanometer.

If the dimensions and number of turns of the galvanometer and the value of H can be measured accurately the reduction factor can be calculated. We shall suppose, however, that these data cannot be directly measured, and turn to another property of an electric current for a means of determining the reduction factor.

Let i be a current which produces a deflexion & in a galvanometer of which the reduction factor is k; then if it be used as a tangent instrument we have

and therefore,

i = k tan 4,

ki/tan p.

If we can find by some other means the value of i, we can determine k by observing the deflexion

produces.

which it

Now it has been found that when an electric current is allowed to pass through certain chemical compounds which are known as electrolytes, the passage of the current is accompanied by chemical decomposition. The process is called Electrolysis; the substance is resolved into two components called Ions; these collect at the points at which the current enters and leaves the electrolytes respectively.

The conductors by which the current enters or leaves the electrolyte are known as the Electrodes1; that at which the current enters the electrolyte is called the Anode, and the component which appears there is the Anion. The conductor by which the current leaves the electrolyte is the Kathode, and the ion which is found there is the Kathion. An apparatus arranged for collecting and measuring the products of electrolytic decomposition is called a Voltameter.

Moreover, it has been shewn by Faraday ('Exp. Res.' ser. vii.) that the quantities of the ions deposited either at the kathode or the anode are proportional to the quantity of electricity which has passed. If this quantity be varied the quantity of the ions deposited varies in the same ratio. This is known as Faraday's law of electrolysis.

DEFINITION OF ELECTRO-CHEMICAL EQUIVALENT.The electro-chemical equivalent of a substance is the number of grammes of the substance deposited by the passage of a unit quantity of electricity through an electrolyte in which the substance occurs as an ion. Thus, if in a time t a current i deposits m grammes of a substance whose electro-chemical equivalent is y, it follows from the above definition, in conjunction with Faraday's law, that

The term 'electrode' was originally applied by Faraday in the sense in which it is here used. Its application has now been extended, and it is employed in reference to any conductor by which electricity enters or leaves an electrical apparatus of any sort.

If, then, we observe the amount of a substance, of known electro-chemical equivalent, deposited in time, we can find the current, provided it has remained constant throughout the time t. If a current be allowed to pass between two plates of copper immersed in a solution of sulphate of copper, the sulphate is electrolysed and copper deposited on the kathode. The acid set free by the electrolysis

The

appears at the anode, and combines with the copper. quantity of copper deposited on the kathode in one second by a unit current has been found to be 00328 gramme. This is the electro-chemical equivalent of copper. The loss of weight of the anode is for various reasons found to be somewhat in excess of this.

We proceed to describe how to use this experimental result to determine the reduction factor of a galvanometer.

Two copper plates are suspended in a beaker containing a solution of copper sulphate, by wires passing through a piece of dry wood or other insulating material which forms. a covering to the beaker. The plates should be well cleaned before immersion by washing them with nitric acid, and then rinsing them with water, or by rubbing them with emery cloth, and then rinsing them with water. They must then be thoroughly dried. One of the plates must be carefully weighed to a milligramme. On being put into the solution this plate is connected to the negative pole-the zinc-of a constant battery, preferably a Daniell's cell, by means of copper wire; the other plate is connected with one electrode of the galvanometer. The positive pole of the battery is connected through a key with the other pole of the galvanometer, so that on making contact with the key the current flows from the copper of the battery round the galvanometer, through the electrolytic cell, depositing copper on the weighed plate, and finally passes to the zinc or negative pole of the battery. Since the galvanometer

For details as to precautions see Gray, Absolute Measurements in

Electricity and Magnetism, p. 169.

LL

reading is most accurate when the deflexion is 45° (see p. 47), the battery should if possible be chosen so as to give about that deflexion. For this purpose a preliminary experiment may be necessary. It is also better if possible to attach the copper of the battery and the anode of the cell to two of the binding screws of a commutator, the other two being in connection with the galvanometer. By this means the current can easily be reversed in the galvanometer without altering the direction in which it flows in the cell, and thus readings of the deflexion on either side of the zero can be taken.

The connections are shewn in fig. 58. B is the battery, the current leaves the voltameter v by the screw м,

entering it at the binding screw N from the commutator c This consists of four mercury cups, p, q, r, s, with two n-shaped pieces of copper as connectors. If p and s, 9 and

respectively be joined, the current circulates in one direction round the galvanometer; by joining and q, r and s, the direction in the galvanometer is reversed. The cup is connected with the positive pole of the battery B.

Now make contact, and allow the current to flow through the circuit for fifteen minutes, observing the value of the deflexion at the end of each minute. If there be a commutator in the circuit as in the figure, adjust it so that See next page.