For charging those a continuous current from the main is most convenient, some simple arrangement to regulate the rate of charge being all the intervention necessary. Such arrangement acts by way of resistance, allowing only the proper amount of current to pass from the main along the accumulator circuit.

Incandescent lamps suitably combined in parallel are com- monly used for this purpose, their capacity being known and their action easily observed. Thus, at 100 volts one 16 candle- power lamp takes J ampere—in other words, lets £ ampere pass on to [charge the accumulator. One 32 candle-power lamp will take 1 ampere, as also will two 16 candle-power lamps in parallel; so that, by arranging the power and number of lamps placed in parallel, an accumulator may be charged at any desired rate.

Fig. 19 shews diagrammatically an arrangement with four lamps in parallel, the positive terminal of the accumulator being connected direct to the positive pole of the supply, while the negative terminal has an ammeter and the lampresistance interposed in its connection with the negative pole of the main.

Any wire of a pair connecting a lamp with the main circuit may be tapped for charging an accumulator by cutting it at some point, and connecting the severed ends to the proper respective terminals. But a more convenient method is to make or obtain some permanent arrangement of lamps, as shewn in Fig. 20. This may be connected with the main by a plug when required, and from its terminals connections passed to the accumulator. With lamps in circuit, the correctness of the connecting, as regards polarity, may be judged from the brightness of their incandescence. When connection is correct, the lamp or lamps will burn dimly, since the residual current of the cells will be opposed to that of the main. If the connections be reversed, the lamp will burn very brightly, since the accumulator current will then reinforce that of the main.

But the method by use of litmus-paper, previously de- scribed, may prove more definite. Once determined, the polarity of the arrangement employed should be marked for future guidance.

Another method of cleaning the emulsion is to wash it in  an open jar placed under a tap. and let the washings settle  for a week.

To avoid this inconvenience, many recent types of break are  constructed so as to employ gas as a dielectric.

(b) The turbine or jet type of break has an action indicated somewhat by its name. When such a break is sot in  motion, a jet of mercury is by centrifugal action propelled  radially from a central stem, and makes contact with a rapid  succession of peripheral sectors of metal, having between  them intervals of insulating material.

The succession of makes and breaks is obtained by a rotary  motion of either the central stem carrying the radial jet with  it, or of the peripheral sectors while the jet is in a fixed direc-  tion. In the earlier and more common designs, speed and  rate of interruption can be directly regulated by altering the  current sent to the driving motor. The metal sectors can be  varied in number to secure a similar regulation, and these  sectors are further made of a triangular shape, so that verti-  cal adjustment may vary the width of metal exposed to the*  jet, and consequently the duration of contact. The surface  of the mercury is in this form of break, as in the dipper type,  covered by some insulating and spark-extinguishing liquid,  paraffin being here usually employed.

For a self-contained portable set we have already recom- mended the combination of dynamo and accumulators, whereby regular X-ray work may be done with only an occasional run of the dynamo. The driving power for the dynamo must be decided according to the special circumstances of each case.

The various methods possible are discussed on p. 45.

The dynamo should be shunt-wound, and should supply current of a higher voltage (4 or 5 volts more) than the total E.M.F. of the accumulator’s strongest discharge.

The charging switchboard should carry a voltmeter and ammeter, and the current should not be switched on to the accumulator till the voltmeter registers the proper voltage. Towards the end of charging the voltmeter will be observed to register much higher, with a corresponding drop in the register of the ammeter.

It is advisable to have an automatic switch inserted in the circuit between dynamo and accumulator, by which, if the dynamo should for any reason stop unexpectedly, the circuit would be at once broken. Otherwise, in such an event, the accumulator would rapidly discharge back through the dynamo, to the probable damage of both.

3. Charging from Public Supply Mains.—Where readily available, no one would seek other source than this for charging accumulators, but the current must be suitably regulated or modified before turning it on to the cells. Con- nection direct to the mains of a supply at any usual voltage would destroy the cells.

The methods of modification necessarily differ according as the supply is continuous or intermittent in character. (a) Continuous Current.—Where installed, continuous current from the main should be used for direct supply to. the X-ray installation, as described earlier, but for bedside work demanding portable apparatus accumulators will further be necessary.

This is usually due to ‘ sulphating’—that is, the formation of lead sulphate in a crystalline form, which may be seen as white patches on the positive plates. In a charged cell at rest there is always some leakage of current, and, as in usual discharge, lead sulphate is formed. This, as first deposited, is soft, and easily altered by recharging; but if that be long postponed the deposit becomes crystalline, and is no longer altered by the current. This deposit reduces the available area of lead, and consequently decreases the capacity of the cell. Plates much affected become useless, and should be replaced by new ones.

The obvious remedy is frequent recharging*. Where insoluble patches have already formed, they should be scraped off with a piece of glass or other non-conductor.

Some try to prevent sulphating by adding 1 ounce of caustic soda to 5 gallons of the electrolyte, but careful working should obviate trouble from this source.

Treated properly, a set of accumulators should do good service for many years, but the various points mentioned must be constantly attended to.

Accumulators may be charged by—

Each of those sources of supply is discussed elsewhere in this chapter, and we need only note here their suitability and method of use for the present purpose. The foregoing notes on charging are applicable to all.

1. Primary batteries need hardly be mentioned, since no serious worker would suggest their use for other than toy installations, and even there the batteries might better be connected direct to the coil.

2. Charging by dynamo is the usual method adopted to charge accumulators where no main supply is available; and, even where the latter source is available, unless it may be ntilized without special transport of the accumulators, we recommend the former.