of the copper oxide in the Edison-Lalande battery, for, since that is mixed with a certain amount of the unreduced oxide, its value may be less than that assigned to it in Table XIII.

The third advantage claimed for the galvanic cell— viz. that it is a very efficient transformer of chemical energy into electric energy—has not only a real existence, but is of the greatest importance. At present, however, this high efficiency of the cell has a scientific interest only, since, although 95 per cent. of the energy represented by the chemical action in a cell may be usefully employed, we cannot with any known form of galvanic cell produce a Board of Trade unit with a consumption of less than about 14 lb. of zinc, which costs not less than 31d., when allowance is made for casting and amalgamating; further, a sum of 8d., or so, has to be expended per Board of Trade unit for the oxidising and depolarising agents, if the waste product be regarded as practically valueless.

Hence, it is impossible at present for cells to compete with steam-engines and dynamos for the lighting of towns, since, although 95 per cent. of the energy contained in the coal may be entirely wasted with our present methods of distributing it electrically, a Board of Trade unit delivered to a house represents a combination of at the most 3lbs. of coal, which cost, say, 0.3d., with the oxygen of the air, which costs nothing. Hence, excluding labour, interest on capital, etc., the cost price of a Board of Trade unit delivered to a house now represents under ld.; whereas, also excluding labour, interest on capital, &c., it would represent at the very least 1s. if supplied from cells in the house.

Should, however, a really economical method be devised of dealing with the waste product from some galvanic cell, or, better still, when we have found out how to cause some fuel of the value of coal, or of oil, to combine rapidly in a galvanic cell with some oxidiser as cheap as air or water, then the comparison of the cost

of producing electric energy with cells and with a dynamo will require careful reconsideration.

But it is to be remembered that the solution of this latter problem requires an entirely new discovery to be made, and that the mere cheapening of electric lighting is one of the smallest results that would follow from it. For a method of causing the energy liberated by the oxidation of cheap fuel to appear in an electric or mechanical form without the employment of any heat-engine would entirely revolutionise our existing methods of producing motive power, and might enrich the discoverer beyond the dreams of avarice. (See the note on page 563.)

Until, however, we are able to form some conception of that mechanism in man and in animals by the aid of which the energy of food is turned directly into muscular energy, or until we are in possession of new methods of dealing with such waste products as zinc sulphate, any scheme for using galvanic cells to electrically light towns should be received with considerable scepticism.

149. Measuring a Cell's Resistance when Very Small. A method of measuring the resistance of a battery or cell by using a voltmeter and an ammeter was described on page 366, § 119; and, in the case of cells having a very low internal resistance, this method is very valuable. The resistance of an accumulator, for example, is frequently under 0.001 ohm; hence, until the current is as large as 20 amperes, the P.D. between the terminals of such a cell will not differ from its E.M.F. of 2 volts by as much as 1 per cent.-or, in other words, the fact that the cell has a resistance will not cause a diminution of as much as 1 per cent. in the value of the current for currents under 20 amperes. Hence, it follows that somewhat large currents must be employed in order to measure the resistance of such an accumulator with any degree of accuracy; and while, on the one hand, such currents are liable to change the resistance of a coil by warming it, and so prevent us

being quite sure of the resistance of a coil from previous tests, such currents are very suitable for being measured with an ordinary commercial ammeter.

Consequently, if a suitable voltmeter be also available, the method illustrated in Fig. 169, page 366, which does not require the resistance of the coil x to be known, may be very conveniently used for testing the resistance of a battery or cell intended to produce large currents.

Another reason why large currents must be used in testing the resistance of an accumulator is that this resistance is not a constant, but diminishes as the current increases. Hence, if it be desired to know the resistance for a current of 50 amperes, this current must be employed when making the test.

150. Measuring a Cell's Resistance when Not Very Small. In the case of cells like the Minotto, which have a resistance of several ohms each, the method of measuring the resistance which is described in the preceding section would generally be unsuitable, as it would require the use of an ammeter graduated in hundredths of an ampere. Further, no difficulty is met with in using ordinary resistance coils, since, with such cells, the currents that can be produced are but small.

As with the preceding method, two separate tests must be made, since the E.M.F. of the cell as well as its resistance is generally unknown, and the tests employed must be of such a kind that the E.M.F. can be eliminated from the two equations which express the results obtained on carrying out the two tests in question. For example:

:

Method No. 1 (a).—Let a cell of unknown resistance, c ohms, be used to send a current through a galvanometer of known resistance, g ohms-first, when a resistance of r ohms is inserted in the circuit in addition to c and g; second, when this added resistance is changed to ohms. Let C and C' be the relative

strengths of the currents determined from the two deflections of the galvanometer and from its relative calibration curve. Let E be the unknown E.M.F. of the cell in volts, then

No. 1 (b).—If r and r' be so chosen that C is twice C', then

Method No. 2 (a).—Instead of varying the added resistance, let the galvanometer in the first test be unshunted, and, in the second, be shunted with a shunt of 8 ohms' resistance, and let C and D be the relative strengths of the currents produced, then

No. 2 (b).-Hence, if r be nought, and g be large

compared with 8,

c=

C - Ds.
D

Method No. 3 (a).—Let the galvanometer be shunted with a shunt of resistance s when making both tests, the former being made, however, with a resistance of r ohms in the main circuit, and the second with a resistance of rohms; then, if G and G' be the relative strengths of the currents in the two cases,

No. 3 (b).—If r be made nought and the galvanometer be very sensitive, s may have to be made so small, to reduce the first deflection to a readable amount, that g s 9+8

will be inappreciably small compared with c. In that case

No. 3 (d) therefore, if also be chosen so as to make G' equal to half G,

c = r'.

Method No. 4 (a).—When making the first test, let a resistance of r ohms be in the main circuit, and let the galvanometer be unshunted; while, in making the second test, let r be charged to rohms, and let the galvanometer be shunted with a shunt of resistance s.

Then, if