connections of WB and m to R are of very low or negligible resistance compared with that of R.

Observations.-(1) Connect up as in Fig. 51, and adjust both G and A to zero, the former roughly, and the latter accurately, seeing that its constant is so

arranged as to make it read 1 amp. for a full-scale deflection.

(2) Make PQ = QT = 5000 ohms, say, and R = 1 ohm, and the rheostat r a maximum.

(3) With K and S open, close m, and accurately measure the value of R, calling it R.; then open m, leaving the WB arms unaltered.

R, R

G

K2

WB

m

S

B

FIG. 51.

(4) With K and m open, close S, and adjust the current to I amp. by means of r, and great care must be taken to keep it

constant.

(5) In manipulating K, tap gentiy, and only for an instant at first; then adjust PQ and QT, so that on closing K1, G does not deflect, PT still being maintained at 10,000 ohms. Note the value r1 of QT.

(6) Adjust PQ and QT, so that on closing K2, G does not deflect, with PT still equalling 10,000 ohms, and note the value 11⁄2 of QT.

3

(7) Repeat (4)-(6) for currents A =,, and amp. respectively.

(8) Calculate for each the E.M.F. of the standard cell C from

Standard cell, ; No.

Temperature of

; (PQ + QT) = ohms; E.M.F., V, = volts approximately. Resistance of R when

Reading of A.

Room. Cell, C. Cold, Ro. Warm. Deflection. True amp.

72. EAR

Inferences. Prove the relation given in (8), and state any assumptions made in obtaining it.

NOTES.-The following is the correction for variation of E.M.F. with temperature for any temperature ° C. :

For Board of Trade or ordinary Clark's cells, E. M.F. = 1'43410'00078(t° 15°)}, legal volts.

=

=

For Carhart-Clark cells, E.M.F. 1'434{1—0′00038(-15°)}, legal volts.

Resist Warm

Corrections. If R is subject to heating, its resistance when warm must be obtained and used in finding Ec. To do this, let the current used, A, flow for a sufficient time to allow R to gain a constant temperature, t,; then suddenly open K and S, and close m quickly, balancing the WB. Note the resistance R1 so obtained, and the number of seconds after the "break," when this balance was accomplished. Repeat this for a longer and shorter time if possible. Then, if Rt, Rt, . . . are the resistances of R at the times t1, tą, . . . secs. after the break, the resistance "warm" at the instant of the break is found as indicated in Fig. 52, and is equal to the ordinate NQ.

FIG. 52.

R

64. Effect of Size and Distance between the Plates of a Cell on its E.M.F. and Internal Resistance.

Introduction.-Relative measurements of different E.M.F.'s may be made by employing a galvanometer of high resistance compared with that of the source of E.M.F., and one in which the scale deflections are proportional to the current-strengths, as, for instance, in a tangent, D'Arsonval, or mirror galvanometer respectively. Internal resistance may be conveniently measured by what may be termed the "fall of potential" method (vide p. 77), using such a galvanometer as the above.

Apparatus.-Reversing switch, K1; high-resistance galvanometer, G; variable known resistance, R; key, K; cell, B, to be tested, consisting of a Daniell's cell, so arranged that the plates

can be raised or lowered in the liquid, and also placed at various distances apart (vide p. 339).

G

Observations.-(1) Adjust the galvanometer needle to zero, and place the plates as near together and as deep in their liquids as possible. With K, unclosed and manipulating K1, obtain deflections on both sides of zero. Denote the mean of these two by d. (2) Repeat (1) with two-thirds and onethird of the plates immersed and the tips just in respectively.

(3) Separate the plates to one-third of the total length of cell, and repeat (1) and (2) for this new position.

(4) Repeat (3) with the plates two-thirds and the maximum distance apart.

Κι

R

FIG. 53.

B

(5) Adjust R to such a value that on pressing K, an appreciably smaller deflection is obtained on G than hitherto. Now note the mean deflection d on pressing K, for each of the positions of the plates mentioned in (1)-(4).

(6) Calculate the internal resistance of the cell for each position from the formula

N.B. If a tangent galvanometer is being used for G, and the deflections are in degrees, the tangents of these must be taken.

Inferences. Write out carefully all the inferences which can be drawn from the above experimental results, and point out their bearing on the construction of cells intended for large currents. Prove the formula given in (6), and state what assumptions are made in deducing it.

65. Determination of the Relation between E.M.F. and Temperature of a Thermoelectric Element.

Introduction.-Whenever contact is established between two dissimilar metals, an electric P.D. is set up between their "free" extremities. Thus, if a rod of some metal, such as bismuth, for instance, has two copper wires soldered to its ends, which are at different temperatures, a P.D. is set up between the free ends of the copper wires, which will produce a current whenever they come in contact. Such an arrangement as this is commonly called a "thermo-couple," and a series of such, in which the two sets of alternate junctions are at different temperatures, constitutes a "thermo-pile." The magnitude of this effect depends on the difference of temperature of such junctions, and the present experiment is arranged with the object of investigating the relation between the two.

Apparatus.-Thermo-couple to be experimented upon (p. 339); sensitive galvanometer of fairly large resistance (p. 280); and a resistance box.

Observations.-(1) Connect this apparatus in simple series, and adjust the galvanometer to zero.

(2) With both copper cans nearly full of cold water, heat up the right-hand one to boiling point, as shown by its thermometer, the temperature of the other being kept at that of the cold water, and as constant as possible.

(3) Adjust the resistance so as to obtain a full-scale deflection on the galvanometer when the water in the right-hand can boils freely. Note this deflection d, which is proportional to the E.M.F. of the thermo-couple and the temperatures f, of the hot and to of the cold water.

(4) Remove the flame, and take simultaneous readings of t1, t, and d every 5° C. down to the lowest temperature obtainable, keeping the circuit resistance constant all the time, and tabulate as follows:

to ensure uniformity of temperature.

(5) From the total circuit resistance, deflections, and the "figure of merit" (p. 28) of the galvanometer, calculate the E.M.F. of the thermo-couple in micro-volts.

(6) Plot a curve having values of d or E.M.F. as ordinates, and values of (t1 − t) as abscissæ.

Inferences. State clearly all that can be inferred from the results of your observations. Why should the circuit have a high resistance preferably?

66. Preparation of the Clark Standard Cell..

Definition of the Cell.-The cell consists of zinc and mercury in a saturated solution of zinc sulphate and mercurous sulphate in water, prepared with mercurous sulphate in excess, and is conveniently contained in a cylindrical glass vessel.

Preparation of Materials. -The Mercury.-To secure purity it should be first treated with acid in the usual way, and subsequently distilled in vacuum.

The Zinc.-Take a portion of a rod of pure zinc, and solder to one end a piece of copper wire. Clean the whole with glass paper, carefully removing any loose pieces of zinc. Just before making up the cell, dip the zinc into dilute sulphuric acid, wash with distilled water, and dry with a clean cloth or filter paper.

The Zinc Sulphate Solution.-Prepare a saturated solution of pure (recrystallized) zinc sulphate by mixing in a flask distilled water with nearly twice its weight of crystals of pure zinc sulphate, and adding a little zinc carbonate, in the proportion of about 2 per cent. by weight of zinc sulphate crystals, to neutralize any free