acid. The whole of the crystals should be dissolved with the aid of gentle heat, i.e. not greater than 30° C., and the solution filtered while still warm into a stock bottle. Crystals should form as it cools.

The Mercurous Sulphate.-Take mercurous sulphate sold as pure, which is white, and wash it thoroughly with cold distilled water by agitation in a flask; drain off the water, and repeat the process at least twice, but after the last washing, drain off as much water as possible. Mix the washed sulphate, in the proportion of about 12 per cent. by weight of ZnSO,, crystals with the zinc. sulphate solution, adding sufficient crystals of zinc sulphate from the stock bottle to ensure saturation, and a small quantity of pure mercury. Shake them well up together to form a paste of the consistency of cream. Heat the paste sufficiently to dissolve the crystals, but not above 30° C. Keep the paste for 1 hour at this temperature, agitating it from time to time, and then allow it to cool.

Crystals of zinc sulphate should then be distinctly visible throughout the mass. If this is not the case, add more crystals from the stock bottle, and repeat the process. This method ensures the formation of a saturated solution of zinc and mercurous sulphates in water. The presence of the free mercury throughout the paste preserves the basicity of the salt, and is of the utmost importance. Contact is made with the mercury by means of a platinum wire about No. 22 B.W.G., which is prevented making contact with the other materials of the cell by being sealed into a glass tube, the ends of the wire projecting beyond those of the tube. 'One end forms the terminal; the other end, and part of the glass tube, dip into the mercury.

To set up the Cell.-The cell may be conveniently set up in a small test tube of about 2 cms. in diameter and 6 or 7 cms. deep.

Place the mercury in the bottom of this tube, filling it to a depth of say 15 cms.

Cut a cork about 0'5 cm. thick to fit the tube. At one side of the cork bore a hole through which the zinc rod can pass tightly; at the other side bore another hole for the glass tube which covers the platinum. At the edge of the cork cut a nick through which the air can pass when the cork is pushed into the tube.

Pass the zinc rod about 1 cm. through the cork. Carefully clean the glass tube and platinum wire, then heat the exposed end of the wire red hot, and insert it in the mercury in the test tube, taking care that the whole of the exposed platinum is covered.

Shake up the paste, and introduce it without contact with the upper part of the sides of the test tube, filling the tube above the mercury to a depth of rather more than 2 cms.

Now insert the cork and zinc rod, allowing the glass tube to pass through the hole in the cork made for it.

Push the cork gently down until its lower surface is nearly in contact with the liquid. The air will thus be nearly all expelled, and the cell should be left in this condition for at least 24 hours before sealing, which should be done in the following way :

Melt some marine glue until it is fluid enough to pour by its own weight into the test tube above the cork, using enough to cover completely the zinc and soldering. The glass tube should project above the top of the marine glue.

The cell thus set up may be mounted in any desirable way; do it so that the cell is immersed in a water-bath up to the level say of the upper surface of the cork. Its temperature can then be determined more accurately than is possible when the cell is in air.

67. Action of Shunts on Galvanometer Deflections.

Introduction. It is often the case that a certain galvanometer G, which is the most suitable one to use, happens to be too sensitive, and that the current which it is desired to measure produces a deflection quite off the scale. If the galvanometer is of the suspended needle type its sensibility might perhaps be sufficiently reduced by bringing the controlling magnet close to the needle. The best method, however, is to connect the galvanometer terminals by a suitable resistance, which allows a certain fraction only of the main current to go through G, the rest through the

"by-pass" or "shunt" circuit, as it is called. It is sometimes the case that the insertion of a shunt so reduces the resistance between the terminals of G, and therefore that of the circuit, that no effect is produced on the deflection owing to increase of total current. Thus the precise action of shunts on galvanometer deflections for different conditions of the circuit is a most important matter. The more elaborate forms of galvanometers are provided with shunt boxes fitted usually with three resistances respectively,,, and, that of G thus making it possible to send only, and of the total current through G respectively. If s = the resistance of the shunt, and g that of the s + 8 galvanometer, then is called the "multiplying power" of the

S

1

shunt, and is the amount by which the galvanometer current must be multiplied in order to obtain the total current in the main circuit.

Apparatus.—Galvanometer, G; resistance box, R; battery, B, of one or more cells; key, K; resistance box, S, for use as a shunt; plug or other similar key, K1.

Observations.-—(1) Connect up as indicated in Fig. 54, and

G

B

FIG. 54.

adjust the galvanometer needle to

zero, using one cell.

(2) With K, open, close K, and adjust R to as high a value as possible, preferably not less than say a hundred times the galvanometer resistance g, so as to get nearly a steady full-scale deflection d. Note this and the value of R.

(3) Close K and K,, and adjust S, keeping R as before, so as to obtain about d less deflection than before. Note this d, and the value of S.

(4) Repeat (3) for about ten different deflections, decreasing by about equal amounts by altering S, keeping R constant, and

show that the relation =

d1 S
holds for each,
d s+ 8

tan d tan d

being used

in the case of a tangent galvanometer.

(5) Repeat (2)-(4) for two cells, reproducing the original

deflection d, and tabulate as follows:

(6) Plot curves having values of s as abscissæ, and currents d, through G as ordinates.

Inferences.—Prove the relation given in (4), and state what assumptions are made in obtaining it. What inferences can you

deduce from the results of your experiments? Prove that = resistance of the shunted galvanometer.

sg

s+ g

68. Relation

between the

Current in a

Shunted Galvanometer and the Total
Current in the Main Circuit.

Introduction.-When the resistance of a galvanometer is small compared with that of the rest of the circuit, any alteration in its effective resistance will practically not alter appreciably the total circuit resistance, and consequently the current. Now, we have seen that the effective resistance between the galvanometer sg terminals is reduced from g to on shunting it with a shunt s, s + g

and therefore if the diminution of this resistance, viz. g

s+g

is at all comparable with the resistance of the rest of the circuit, the total circuit resistance, and hence the total current, will be considerably altered (increased). The object therefore of the following experiment is to see what effect such an increase of total current has on the galvanometer current, and the deflection it produces, and the conditions under which this effect takes place. It will be obvious at first sight that the question is one of great practical importance.

Apparatus. Precisely that mentioned in the preceding test,

with the addition of another galvanometer, Gm, for the main circuit.

Observations.-(1) Connect up as in Fig. 54, placing G, in the main circuit, and adjust both galvanometers to zero, using one or more cells of fairly constant E.M.F.

(2) With K, open, adjust the battery E.M.F. and R (which should preferably not be more than two or three times g) so as to obtain about a half-scale deflection, d, on the main galvanometer Gm. Note this, and also the value of R, and the deflection d on the other galvanometer G.

(3) Close K, and K, and, with R as before, adjust S to about double the resistance of G. Note the deflection d of G and dm of Gm and S.

(4) If the deflection d has altered, adjust R only, so as to reproduce the first deflection obtained in (2) above, and note the value R, required to do this. Then reduce the resistance again to its original value, as in (2) above.

(5) Repeat (3) and (4) for about ten different values of s, decreasing by about equal amounts to say one-tenth of the value of the galvanometer resistances G.

Tabulate your results as follows:

:

(6) Plot the following curves, one having values of s as abscissæ and currents d through G as ordinates, the other with d as ordinates and

sd...

s+8

as abscissæ.

Inferences.-State concisely all that can be inferred from your experimental results.