Renard batteries belong to the chromic depolarising class, in which free chromic acid and hydrochloric acid, more or less diluted or mixed with sulphuric acid, are used. Each element consists of a cylindrical ebonite tube, a platinised silver electrode 1mm. thick, rolled into the form of a tube, and a non-amalgamated zinc cylinder of a diameter about 1/6 that of the vessel. The complete substitution of hydrochloric for sulphuric acid has permitted of quintupling the specific power of the element. On making but a partial substitution of hydrochloric for sulphuric acid, we obtain attenuated liquids that give the same quantity of total electric energy, but with a specific discharge so much the smaller in proportion as the attenuation is higher.

The liquid may be prepared with pure or crystallised or with commercial chromic acid. As the non-attenuated liquid is unstable and disengages chlorine, even at the ordinary temperature, it is prudent not to make the mixture until two days before it is to be used. Mixtures attenuated to 80 per cent, are more stable, and can be prepared 2-3 months previous to use. All the liquids are prepared by mixing three elementary liquids in variable proportions. Liquid A is a solution of chromic acid containing, per litre, 530 c.c. chromic acid and 770 c.c. soft water. Liquid BC1 is a solution of commercial hydrochloric acid made to indicate 18° B. Liquid BS is an aqueous solution of commercial sulphuric acid marking 29° B (density 1.2515). It is obtained by mixing 450 grm. of sulphuric acid of 66° B. with 800 c.c. water. The mixture of the two latter liquors forms an intermediate liquid called sulpho-hydro-chloric, which is so much the richer in BC1 liquid in proportion as one desires to obtain a greater specific power. The letter B, followed by an index, designates a mixture of the two last liquids containing 1 per cent, in volume of sulphuric solution shown by the index.

Thus, for example the liquid B 80 contains 80 volumes of liquid BS and 20 of liquid BC1. The figure 80 bears the name of degree of attenuation. The liquid used in the battery consists of equal volumes of the liquid A and a mixture of the two others.

Whatever be the degree of attenuation, the electric energy obtained per litre of liquid is sensibly the same. It varies between 50 and 60 watts-hour per litre. The duration of the discharge is more or less prolonged, according to the degree of attenuation. This attenuation may be obtained with other products, sulphate of soda, for example; but the specific energy is thus reduced. On arresting the discharge at the moment in which the intensity of the current falls to one-half of the maximum is obtained from 180,000 to 196,000 joules per litre of liquid, and 144,000 to 158,000 joules per kilo. On endeavouring to increase the total electric energy, Renard has devised a liquid that gives as many as 253,000 joules per litre, which would reduce the necessary volume of liquid to 14*5 litres per kilowatt-hour. But as this liquid contains 100 grm. CrO, and 200 c.c. per litre, it has the drawback of being very dear and slightly viscid, and of making the zincs sticky. Practically, the best liquid is that which contains 200 c.c. HCI and 60 c.c. CrO,.

The battery generally has the form of a cylinder of a length 10 times its diameter. This form has the advantage of facilitating the cooling of the liquid, of diminishing the internal resistance, and of rendering the upsetting of the liquid more difficult. In the light batteries the reservoirs are of ebonite; in the stationary ones they are of glass or porcelain.

In the pneumatic battery, designed, for lighting (Fig. 93), the elementary vessels A are sealed to the cover of a large, tight vessel H, and the lower part contains an orifice O of small diameter. Upon forcing air into the large vessel or collector by means of a rubber bulb d, or of a pump, the liquid is made to rise in all the elements at once. A cock permits of regulating the immersion of the elements, and consequently, the internal resistance of the current. This arrangement is adapted to attenuated liquids only; with the normal liquid, the cooling would not proceed quickly enough. The vessel L and the bulb d serve to fill and empty the battery. The positive electrode of the light batteries (Fig. 94) is formed of a tube of platinised silver 1mm. thick. The weight of the platinum on the two surfaces is about 1/10 that of the silver, and its thickness is about 1/400mm. The use of platinised silver greatly reduces the weight, the volume, and the internal resistance of the elements. On account of the high price of these electrodes, carbon is substituted for them in the batteries in which . attenuated liquids are employed and in which lightness does not play an essential role.

In order to facilitate the free circulation of the liquid, and to exhaust all the supply contained in the cylindrical vessel, the tube is split throughout its entire length to a width of a few mm.

The negative electrode consists of a cylinder of zinc or non-amalgamated zinc wire, whose diameter is about 16/1000 that of the vessel, and is calculated to serve but once. This is guided and held in the centre of the platinised silver tube by several discs of ebonite. Experience baa demonstrated that in chlorochromic liquids, ordinary zinc is not so readily attacked as amalgamated zinc, as soon as the proportion of chromic acid exceeds 180 grm. pel Co. of solution A. The amalgamation it costly, and renders the zinc brittle, and its suppression permits of the use of lead in the pneumatic batteries. This could not be done with amalgamated zinc, for the drops of merenry, flowing accidentally over the lead, would soon perforate the envelope.

The discharge of a chlorochromic battery by no means presents the characters of a perfect one. The variation! In the temperature of the liquid, its alteration, and the diminution in diameter of the zinc daring the discharge act so as to modify the constants of tbe elements at every instant. On another hand, the liquid of the battery exerts an important local chemical action entirely independent of the electric action, and in such proportions that it from the liquid when the battery is not in lervice.