The voltage of 1.9, so often mentioned here, is only that to which makers recommend their (ells lK?ing discharged. In some instances, a figure as low as 1.8 is stated to be that to which a particular make of cell will discharge, but as it is only a question of how low the cell can be discharged without injury to it. one can understand it is not advisable to "cut it too fine".

The charging of cells, unless efficiency is of great importance compared with durability (as is seldom the case), is continued at a maximum current decided by the makers, until the cell "boils", - in other words, until the electrolyte or acid gives off gas so freely as to give the cell that appearance. The temperature of the cell is not to be thought to have increased by the so-called " boiling" of the electrolyte. on the appearance of "boiling" in all the cells, the charging is discontinued, and the total E.M.F. will then, if the cells are healthy, be generally found to be represented by 2.2 volts per cell. With glass boxes for the plates, the "botting" will be found to give a "milky" appearance to the acid, until the charging ceases,

Much spray is given off during: the "boiling", which occurs fortunately only towards the end of the charging, and is a sign that the plates are approaching the desired condition. To prevent this spray, many devices have been thought of. One is to lay a piece of glass over the top of the glass box, in order to catch and throw beck the spray. To effect this, the glass should he slightly raised at one end, so that the spray accumulating thereon may drop beck into the box. Care must be taken that these glass plates do not touch both the negative and positive plate in any box, as, should they do so, a path would be formed for tin-current from the negative to the positive, apart from its legitimate circuit. To show the objection to this, it is only necessary to remember that a cell consists of a number of positive and negative plates interleaved with one another, but separated from each other with scrupulous care, the only connection being by means of the liquid. If something were to connect the positive and negative terminals outeide the box (as the wet sheet of glass would do), the cell would have a circuit from (say) the positive plate through the liquid to the negative plate, and thence along the wet sheet of glass back to the positive plait*-.

Another method is to till the glass box within about half an inch of the top with the liquid, - usually the liquid need only be so high as to well cover tie-plates, - and then to pour upon it hot paraffin wax of the best quality to a depth of the remaining half-inch, having previously fastened in one corner, for the moment, a greased wood plug, which can then be removed, leaving a hole in the-wax of half an inch in diameter, through which a little acid can be withdrawn by a syringe. This syringe, or any such tool used with accumulators, must not be of metal, but of glass or other insulating material, for fear of accidentally connecting the positive and negative plates, which would have the effect of the wet sheet of glass above referred to, but with far more disastrous result.

This arrangement of wax is very effective as regards preventing the spraying of the acid, and if the level of the liquid is reduced with the syringe sufficiently to allow the gas formed to escape through the hole made by the temporary plug, all will be well, but the plan has the decided disadvantage of preventing the removal of the plates, or the giving of attention to them to dislodge any scale forming between them (as is often found necessary), without the removal of the wax.

Another plan, and one which I consider by far the best, is to allow to float on the top of the acid a quantity of prepared crumbled cork, or small glass balls. These are easily removed, and neither of them can do harm, provided the cork is prepared to withstand the action of the acid.

Anything which will carbonize and become water-logged is, of course, objectionable, as in time the effect might be, that enough would sink to build up a connection between the plates, and so bridge across the positive to the negative.

To read the EM F. of the cell, as mentioned on page 252, hand-voltmeters are often employed, which are generally fitted with a piece of double flexible wire and a "spear", as shown in tig. 622. The spear is about a foot long, and the free ends of the attached wires arc provided with blades, one of which engages in the positive and one in the negative plate, so as to ensure a good contact whilst the reading is being taken on the voltmeter. The contacts with such a spear are however, very treacherous, owing to the oxidized condition of the surface of the lead strips leading to the plates in the cell.

Fig. 622    Hand voltmeter and spear.

Fig. 622 -- Hand-voltmeter and spear.

and care should be taken to make the blades thoroughly scratch the lead, or an incorrect reading will be the result.

These hand-voltmeters arc only suitable for making comparisons; for instance, if all the cells but one read about 2 volts, and this one only reads 15 volts, there is no doubt of the cell being faulty. The hand-voltmeter is quite likely to read to 3 volts per cell, but one must not 1m? led to believe the cell is giving that E M,F. but merely to use the figure for comparison with the other cells. Above all, the E.M.F. of a cell should not be taken, and the figure multiplied by the number of cells; for instance, if the cell above-mentioned should read 8 volts on the hand-voltmeter, and there should be 50 cells, it should not be concluded that the voltage is 150. On the contrary, no reliance should be placed upon such a figure.

The principle of the voltmeter is very simple, and one form, which will suffice to explain it, is that of an electro-magnet attracting a small piece of iron. The greater the strength of the current, the greater will be the quantity of current flowing round the electro-magnet, and the greater will be the attraction of the magnet for the small piece of iron; consequently, the greater the attraction, the greater distance will the indicating needle move, to which it is fixed.