of (e) quality of carbon, (ƒ) cored and uncored carbons, (g) hissing of the arc.

The relative amounts of power absorbed by the arc itself and by the regulating mechanism should be investigated.

Many of the above tests can only be employed on handregulated lamps.

(34) Measurement of the Internal Resistance of Secondary Cells.

Introduction.

The following method is the best for measuring the working value of the internal resistance of a storage cell or battery of such. Owing to the very low resistance met with usually in this kind of cell the ordinary methods are practically inapplicable, and in the present case the cell is being tested more or less under working conditions.

If a battery is being tested the total internal resistance can be obtained at once, and if the cells are all of the same size, make, and type, the resistance of each cell can be deduced, probably with considerable accuracy, by dividing the total resistance so obtained by the number of cells and thus obtaining the average resistance per cell. It should be remembered that the internal resistance of any cell is not a fixed and invariable quantity but depends on several things, thus, for instance, on the density of the sulphuric acid solution which is continually changing according to the amount of discharge, or charge of the cell. It is interesting to note in this connection that the resistance of a solution of dilute sulphuric acid is least at a specific gravity of about 1220 and increases from this in either direction, i.e. for a rise or fall in density. Again, the internal resistance will depend on the condition the plates are in, and will be greater if they are "sulphated" than if in good condition.

Apparatus. The cell or cells (B) to be tested; voltmeter V of sufficiently large resistance, and having a long open scale, enabling small differences to be read accurately; ammeter A capable of reading up to the maximum current to be taken from the cell; key K; switch S; carbon rheostat R (p. 393).

Observations. (1) Connect up as in Fig. 30, and adjust the

pointers of V and A to zero, levelling the instruments if

necessary.

(2) With S open, close K and note the reading E on the

(5) Calculate the working value of the internal resistance b of the cell or battery from the relation

(6) Plot a curve having values of (b) as ordinates and A as abscissæ.

(35) Measurement of the Efficiency and Storage Capacity of Secondary Cells. Introduction. Secondary cells may be divided into two main divisions, namely the "Fauré" or pasted type, and the "Planté

or non-pasted type. The chemical changes occurring in either class, during charge and discharge, are precisely alike, but the reader is referred to ordinary text-books of Electrical Engineering for such changes which hardly come under the scope of the present work.

The secondary or storage cell has taken up so prominent a position at the present day in both electric lighting and electric traction that the method of measuring the efficiency and storage capacity of any type of cell, or perhaps more particularly the relative behaviour of different types under the same conditions, is a matter now of paramount importance to every electrical engineer. A good deal may be said with regard to the precise mode of testing such cells, and in this connection much depends on the duty which they have to perform in actual practice. Any laboratory test of such cells will be worthless almost, from a practical point of view, unless it is carried out under conditions as nearly as possible alike to those the cell will work under in its everyday use. Thus, for instance, take a battery employed for merely lighting purposes, say at a central electricity supply station. It is never resting idle and never merely giving either its full load discharge or any other constant output, for the load which it has to take varies with the hour, day, and season of the year, from often next to nothing, to full load and sometimes a considerable percentage overload for short periods. Thus it will be seen that in this instance any test to be of value must be carried out as nearly as possible under these conditions, and for months continuously, too, instead of, perhaps, only for two or three weeks always at full load and with, say, a night's rest in between each such discharge.

Again, in the case of electric traction work, the above remarks do not all apply, for instance, usually a battery used in this kind of work in sub-stations is relieved of discharge between midnight and about 7 a.m. in the morning when it is charged. When used for portable work, as in autocars and tramcars, it is subject to both rapid and wide fluctuations of output and often to excessive jolting. Hence the test on a cell required for this kind of work should be a very stringent one, automatic jolting gear being provided to operate on the cell while being discharged, while this latter must often be abnormal. Practically the Fauré or pasted

type of cell is the only one available for self-contained autocar traction, as weight forbids the use of the Planté type. As one instance of a traction type of pasted cell which will stand periods of excessive discharge and the wash of the solution against the plates and yet have a long life, the Headland secondary cell may be instanced, and tests extending over years amply justify this. The efficiency of any secondary cell or battery can be reckoned in one of two ways, namely, the

Quantity efficiency, or Ampere-hour efficiency

=

Ampere-hours given ont

Ampere-hours put in

Energy efficiency, or Watt-hour efficiency

Watt-hours given out

Watt-hours put in

Each of these will depend to a certain extent on the relative periods of charge, rest, and discharge, and also on the current density or rate of discharge reckoned say in amperes per unit of area of positive plate. The greater this is the less will be the quantity efficiency, and also the energy efficiency, though not to the same extent.

It may here be remarked that the quantity efficiency may be as high as 94% when the current density is low and the cell used under favourable conditions, whereas the energy efficiency cannot exceed 80% from the fact that the average normal voltage of a cell on discharge is 20 volts approximate and the average voltage needed to charge being 25 about. These two efficiencies in practice may be taken more nearly as about 75% and 65% respectively.

The CAPACITY of any secondary cell may be expressed in one of two ways, namely, either as the ampere-hours or as the Watthours which it is capable of giving as a useful discharge. The term commercial capacity might be given to the number denoting the ampere-hours or the Watt-hours per lb. of plate (taking both and ve together) or per lb. of cell complete, including acid, etc.

ve +

At the present day, owing to there being so many forms and methods of building, the latter mode of reckoning the capacity is the only one available when comparing different types of cells.

A secondary cell may be charged either (1) at constant P.D. or (2) at constant current. In the first case a fairly heavy rush of current takes place at starting, and the method would be unsuitable for use on some types of pasted cells from the risk of the plates buckling. The second method is the one nearly always employed in practice and is the one which will here be considered.

The cell should not be discharged normally below 1.80 volts on closed circuit, since it will then become practically useless for lighting circuits, and there is also the danger of the plates "sulphating" rapidly below this limit. For the latter reason it should not be allowed to rest in this discharged condition. Apparatus.-Cell B to be tested; sensitive voltmeter (V) with

open scale; ammeter (4); switch S1; carbon rheostat R (p. 393); two-way switch S; source of charging E.M.F. (E); hydrometer and weighing arrangements if the latter should be required.

Observations.—(1) Assuming that the cell to be tested is not already set up, but is still as received from the makers. First weigh each complete set of plates, "Positives" and also "Negatives," separately after dusting them. Also weigh the containing vessel, and the dilute sulphuric acid solution (of the specific gravity authorized for that particular cell), which is sufficient to cover the plates and be about one inch above their tops. Measure the size and thickness of the plates.

(2) Set up the cell properly, connecting up as indicated in Fig. 31, and adjust the pointers of Vand A to zero if necessary.