regular catalog discharge rate is about 1.95, and the negative when the cadmium reading is approximately 0.25 volt. At these values. the cell voltage would be 1.7.

As a cell discharges the positive cadmium reading decreases, while the negative cadmium reading will increase. The curve shown in Fig. 22 illustrates the discharge of a cell in which the positive plates are low in capacity. Note the rapid drop in cell and positive cadmium voltage after three hours but the slow rise

Fig. 22.-Curves Illustrating the Discharge of a Cell During a Cadmium Test in Which the Positive Plates are Low in Capacity.

in the negative cadmium voltage. All voltage readings must be taken while the battery is discharging at its catalog discharge rate. The readings should be carefully filed and compared with previous tests. In this way any cell not in condition will be readily found.

Making Electrolyte.-Electrolyte, as used in all lead plate types of batteries, consists of a mixture of pure sulphuric acid and

distilled or other pure waters. Concentrated sulphuric acid is a heavy, oily liquid, having a specific gravity of about 1.835. A battery will not operate if the acid is too strong, and it is therefore diluted with sufficient pure water to bring it to a gravity of 1.270 to 1.300 for a fully charged battery. Stronger electrolyte than this is injurious. To prepare electrolyte from sulphuric acid of 1.835 specific gravity, mix with water in the proportions indicated in Fig. 23 for the desired specific gravity, taking the following precautions:

Use a glass or earthenware vessel, never metallic.

Carefully pour the acid into the water, never water into acid. Stir thoroughly with wooden paddle and allow to cool before reading the gravity.

Both the water and the sulphuric acid used in making electrolyte should be chemically pure to a certain standard. This is the same standard of purity as is usually sold in drug stores as "CP" (chemically pure), or by the chemical manufacturers as "battery acid."

Electrolyte made from sulphuric acid meeting the following specifications will be satisfactory:

Sulphuric acid to be high grade, either the so-called "Brimstone Oil of Vitriol" or "Contact Process Acid" made from sulphur of good quality. Must be water white in color and show no sediment on standing.

By analysis, impurities must not exceed the following:

Must be free from all substances other than stated above. As great care is exercised in the manufacture of storage battery plates and in the furnishing of acid to secure a high degree of purity, obviously attention should be paid to the purity of the water used both in the dilution of concentrated acid, if this is

Fig. 23.-Chart for Preparing Electrolyte of Any Specific Gravity.

1.300

attempted, and replacing the loss in cells occasioned by evaporation and overcharge. The frequent addition of water required to replace evaporation leads eventually to a concentration of any impurities which it may contain. As ordinary water supplies are not pure, their use is always questionable. Water from natural sources should be used only with the approval of competent chemists. Rain water should not be used if distilled water is available, as it often contains traces of nitric acid and ammonia, either of which is harmful to storage batteries. Distilled water is preferable and should always be used unless otherwise advised by the chemists. The water should be stored in carboys or thoroughly cleaned. whiskey barrels. Water obtained by condensing the exhaust from engines and which may thus contain cylinder oil and other impurities should never be used for battery purposes. In cleaning batteries the ordinary tap water may be used provided it does not contain a great quantity of impurities.

In this connection, the expression "chemically pure" acid is often confused with acid of "full strength." Acid may be of full strength (approximately 1.835 sp. gr.) and at the same time chemically pure. If this chemically pure acid of full strength be mixed with chemically pure water, the mixture would still be chemically pure, but not of full strength. On the other hand, if a small quantity of some impurity be introduced into chemically pure acid, it would not materially reduce the strength, but would make it impure. The usual method of determining the strength of electrolyte is by taking its specific gravity. The method is possible on account of the fact that sulphuric acid is heavier than water. Therefore the greater the proportion of acid contained in the electrolyte the heavier the solution or the higher its specific gravity. By specific gravity is meant the relative weight of any substance compared with water as a basis. Pure water, therefore, is considered to have a specific gravity of 1, usually written 1.000 and spoken of as "ten hundred." One pound of water is approximately one pint. An equal volume of concentrated sulphuric acid (oil of vitriol) weighs 1.835 pounds. It therefore has a specific gravity of 1.835 and is spoken of as "eighteen thirty-five."

Since electrolyte, like most substances, expands when heated, its specific gravity is affected by a change in temperature. For the convenience of the operator, the following table is given to show the variation in the electrolyte specific gravity at various temperatures likely to be met with in service:

The solution must be allowed to stand several hours to cool. Never add hot or even warm electrolyte to a cell, as the plates. are liable to be dangerously sulphated thereby. The strength of the resultant solution should always be checked by hydrometer readings reducing the latter to 70 degrees Fahr.

Features of the Edison Cell.-The instructions given apply only to batteries of the lead plate type and not to the Edison battery, which is entirely different in construction. The Edison cell uses an electrolyte consisting of 21% solution of potash in distilled water so that the electrolyte is alkaline instead of acidulous. For 6-volt ignition and lighting service it is necessary to use 5 cells owing to the lesser voltage of the Edison batteries. The average voltage during discharge is but 1.2 volts per cell, and is not as constant as is the case with a lead battery, the voltage of which may be as high as 2.5 volts per cell.

An Edison 6.5-volt battery used for lighting or ignition may be charged completely in ten hours. A feature of the Edison battery is that overcharging at the normal rate has no harmful effects, and it is advised by the maker to give the battery a 12hour charge once every 60 days or when the electrolyte is replenished. The electrolyte must be kept sufficiently high so as to