Where portability is a main consideration—as for field- service—various special adaptations may be employed. The dynamo itself should be of as light a pattern as may be compatible with efficiency, and may be constructed of detach- able sections if difficulties of transport make that advisable. Driving-power may be derived from one of the traction- engines or motors now so generally employed in transport; or a special motor might readily be designed to transport the dynamo and other X-ray outfit, and also serve as driving- power for the dynamo when so required. An automobile of this description, recently designed for field service in the French army, is illustrated in Fig. 26.

A still more portable outfit may be designed for driving by horse-power, as shewn in Figs. 27A and 27B. Where men are readily available—as in the services—a pedal gear may be arranged, similar to the driving gear of a bicycle. In emergency, a serviceable drive may be obtained by supporting an actual bicycle frame, and connecting the dynamo by belt to the back wheel.

For screen-work they produce brilliant, steady illumination of a suitable tube, and are for this purpose excellent. For photography, a tube so excited requires a long exposure, but very good radiograms are produced. If such work be attempted, a tube specially made had better be employed.

Absence of reverse currents, and the improbability of over- heating, prolong the life of the tubes considerably.

As mentioned at the beginning of this chapter, the static machine is a favourite source in other countries, and in this country there is a probability of an improved form coming into favour rapidly when it is introduced, as we expect it to be shortly. At the present stage, however, it would not be profitable to do more here than refer readers to works on static electricity, where those machines are specially described and considered.

A dynamo-electric machine (briefly termed a dynamo), suitably driven, forms a valuable source of supply where such must be instituted in the absence of, or independent of, a general supply. Thus, on board a ship in which electric light is not installed, in hospitals in isolated or country dis- tricts, or specially adapted for field service, this type of supply has much in its favour.

The dynamo may be constructed to supply current suitable for the special purpose in view, and some amount of regula- tion will be possible for variation of speed, though for each machine there is a certain rate of speed at which the greatest efficiency of action is obtained.

The choice of a special form of driving-power will depend mainly on the circumstances of the installation.

Where there is a pre-existent supply of power, with suffi- cient margin, the dynamo should, if possible, be driven from that, either by a direct chain or belt drive, or by way of an intervening countershaft, whereby variation of speed may be obtained, if that be desired.

Where no power-supply exists, and the installation is to be stationary, a small gas or oil engine will usually be the preferable power for driving. Special circumstances may make a steam-engine preferable. Where a sufficient water- power is available and convenient, an economical drive may be obtained from a water-turbine. This plan is highly com- mendable, wherever possible.

Using only one such cell or two in series, it will be readily seen that half of the current will really be lost, since only one of its two periods or phases is transmitted to the accu- mulator. Also the back electro-motive force of the phase suppressed will rapidly heat the electrolyte and wear down the aluminium. Fig. 24, however, shews a plan of connect- ing up two cells whereby both phases of the current may be utilised, the accumulators receiving alternately the single periods or phases as allowed to pass by the cells. By follow- ing out the connections as shewn in the sketch, and noted under it, the action will be more clearly understood than by verbal description. Besides utilising almost the full energy of the current supplied, this arrangement lessens the heating of the electrolyte and the wear of the aluminium electrode, since one or other path is always open to the current, and the back electro-motive force in each cell is thus minimised.

Each accumulator will be charged at half the rate corresponding to the power of the lamp used. Thus, with a 32 candle- power lamp in series, with a 100-volt supply, each will receive about i ampere.

The above arrangement assumes that two or more accumu- lators will be charged at one time. For a single accumulator an arrangement of four cells is possible, arranged after the manner of a Wheatstone bridge; but for X-ray work more than one accumulator will practically always be in use.

III. Primary Batteries.

Primary batteries are of little, if any, practical use in X-ray work. The requisite current can indeed be obtained if a sufficient number of cells be suitably connected; and for such a purpose bichromate cells, or Bunsen’s cells (each with an E.M.F. of about 2 volts), will probably be found most suitable. But it is hard to conceive of any set of circum- stances in which it would not be much more convenient and efficient to employ some other source of supply.

We may repeat that the action of such a cell depends upon a property of aluminium (also magnesium), whereby as kathode it allows current to pass freely, but as anode it offers great resistance. Iron is a convenient material for the other electrode, since it is not acted upon by the electrolyte.

The solution should be a saturated one, and it is preferable to use distilled water in preparing it, as chlorides seem to hamper the action. By escape of ammonia the liquid be- comes acid, and crystals tend to form. To obviate this, a little weak ammonia solution should be added occasionally.

Periodically—each three or four months—the cells should be taken apart, the solution filtered and replenished, and any deposit on the plates or cell scraped off.

In action a certain amount of heat is generated in the cell, and if the temperature rise above 70° F., the efficiency falls.

Thus the size of cell and quantity of electrolyte should be proportionate to the quantity of current passed through the cell; but the aluminium should be kept small in area. The cell should be placed between the lamp-resistance and the accumulator, with the aluminium electrode connected to the positive terminal of the latter.

A cell of about the dimensions described should act well with a current of 100 volts passed through a 32 candle-power lamp. “Where current supplied is at a higher voltage, it will be profitable for regular use to pass it first through a trans- former, whereby pressure will be reduced to 50 volts, and the amperage correspondingly increased; otherwise two cells in series may be employed to deal with the heavier voltage, though this is not so efficient.

Where a plug is employed, it should be of a concentric type, thus making always the same connection with the main. The ordinary bayonet-catch type may be inserted in either of – two positions, and the polarity of the wires from it will differ according to the position. Hence, with such, the posi- tion must be indicated by corresponding marks on the plug and socket, or the polarity of the wires must be tested before each time of using.

Fig. 21 shews a permanent wall-board that would involve a minimum of trouble when once installed. The voltage of supply is there registered, as well as rate of charging, and there is also inserted an automatic cut-out.

(b) Alternating current cannot be used directly for charging accumulators, but some device must be interposed to render the current unidirectional. Many devices have been suggested and employed thus to * rectify’ the current.

Those are mentioned, and some described, earlier in this chapter (p. 27); but, for the purpose of charging accumu- lators, we need only describe the device known, and already referred to, as Nodon’s valve, aluminium cell, or electrolytic • rectifier. In Fig. 17, on p. 30, is illustrated a set of those cells as made for sale, and on the preceding page will be found a brief explanation of the action of the device.

Fig. 22 shows a more simple arrangement of cells connected to an accumulator.

But efficient cells maybe made from simple material by any one for his own use. Thus, a cell may be m ade from a large jam-jar containing a strong solution of neutral am- monium phosphate, and arranged as shewn roughly in accompanying sketch (Fig. 23).

Into the jar dips an electrode of aluminium of about the thickness of a pencil, and a second electrode of thin iron— such as hoop-iron—from 2 to 3 inches broad, each electrode being about 9 inches long. Each electrode is shown suspended in the solution by being passed through a bar of wood or other suitable insulating material which rests on the top of the jar, the aluminium being made firm by passing through a cork and the iron being wedged in. At the top of each is a screw electrode for connecting wires.

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.

They should never be fully discharged in working, the safe limit being indicated by a fall in the voltage of the derived current. Each cell, as mentioned, gives a little over 2 volts when freshly charged, and the bulk of its charge (about 75 per cent.) is given off at that pressure. When the E.M.F. derived from an accumulator falls below 2 volts from each cell connected in series, it is imperative that it be recharged at once.

If, after recharging, an accumulator does not register its normal voltage, test each cell separately with a pocket galvanometer or a 2-volt lamp, so as to discover which ceil or cells are at fault.

Short of serious damage, the failure of a cell to register its full voltage is frequently due to a fall in the level of the acid, caused by leakage or evaporation. (The possibility of leakage points to the necessity of having accumulators placed on leaden trays if they be kept indoors.)

In testing for a fault, do not, as is sometimes foolishly done, spark or flash each cell by connecting its opposite plates by a piece of wire, since such short-circuiting injures the plates.

The chance of such short-circuiting by accidental means must be prevented, a possible danger of this kind in transit having been already mentioned. Thus, in connecting up the induction-coil, fasten the ends of the connecting-wires to the coil before fastening the other ends to the terminals of the accumulator, thereby avoiding the chance of live ends coming into contact. Similarly, it is well to see that the accumulator boxes are not used as a shelf for depositing odd pieces of wire or metal, which may readily bridge the terminals and cause serious damage. In extreme cases, by such short- circuiting, plates may be completely crumbled up.

With careless working, it may soon be noted that an accu- mulator will not absorb nor discharge the certified quan- tity of electricity.