There are two main designs of this type of break, one with  a perpendicular dipper and the other rotary, each being  driven by a small motor on a circuit independent of the circuit  of supply to the coil.

The perpendicular dipper is worked by a crank motion  attached directly to the shaft of the motor. It allows of simple  and exact regulation of the amount of dip and consequent  duration of contact, as well as of the rate of speed. This is a  simple and straightforward mechanism, and does not readily  get out of order. It is also easily cleaned, so that an instru-  ment-maker need seldom, if ever, be called in to assist in its  continued working. A high rate of speed cannot be attained  —about 1,000 to 1,500, and the break is somewhat noisy in  action, but for combined work it acts on the whole very well.  The rotary design of dipper break, usually associated with  the name of Mackenzie Davidson, has an inclined axle, on  which the dipping blade or blades are fixed radially. Those  radial blades make and break contact with the mercury while  the axle revolves. With this break a higher rate of speed can  be attained than with the perpendicular dipper, but the inter-  vention of a belt drive, as in the usual form, is a disadvantage;  while the mechanism is less simple, and may require skilled  assistance more frequently to keep right. More recent forms  have the motor set on an inclined base, and the vanes attached  to a direct prolongation of its shaft, as shown in Fig. 31.  Either of these dipper breaks serve very well for moderate  currents and for medium rates of interruption.

Regulation of the various tension screws should be attended  to, and the platinum contacts kept always in good order, to  insure full efficiency in working.

The vibrating or hammer break is still largely employed in  both army and navy services; but for attainment of best  results, and especially for employing the heavier currents now  found so advantageous, it seems advisable that these breaks  should be replaced by some more recent form.

For the benefit of workers whose practice may be confined  to, or chiefly concerned with, such form of apparatus, we  have, by request, appended to this chapter some notes on the  connections and working of a coil with vibrating break.

2. (a) The dipper break makes and breaks contact by  means of a metallic rod alternately dipping into, and being  withdrawn from, a reservoir of mercury, from which a con-  nection passes to complete the circuit. The mercury is covered  by a layer of liquid, such as alcohol or paraffin, so as more  effectually to quench the sparks produced in action of the  break.

For radioscopy or screen examination we wish a steady fluorescence; hence rapidity of interruption will be the criterion. For radiography the same high rate is not essential, but will lessen the requisite length of exposure. Since those two classes of work are usually combined, we may say that for such work a fairly high rate of interruption is essential, up to 8,000 per minute.

In radio-therapy, on the other hand, there is no especial call for rapid interruption, and a rate of 1,000 per minute will be more than sufficient for such work. The duration of exposure will here frequently be much in excess of those employed in the other classes of work, varying from five to twenty-five minutes, so that unless the apparatus be designed to stand such prolonged runs, it may be unable to withstand the strain.

Where one or other class of work distinctly preponderates, the installation should be designed to suit that work; where neither preponderates, a compromise must be struck, unless Intermediate Apparatus 5′ the installation may be duplicated. Some recent coils are made with arrangements for adjustment to suit varying conditions, and with an interrupter of wide margin of rate may be made to suit the work in hand, but never so efficiently a coil designed specially for specified conditions.

With a knowledge, then, of the nature of the work to be done, the radiologist may settle what rate of interruption will be most suitable, and he will have a coil built to suit; but first he will settle on the interrupter likely to fulfil the conditions.

Interrupters or breaks are in type many and various. They may be classified as—
1. Vibrating or platinum.

2. Motor mercury, including—

(a) Dipper type;

(b) Turbine or jet type.

These two pieces of apparatus—coil and interrupter—should always be considered jointly, and designed mutually to suit each other. Different breaks produce very different rates of interruption, and no single coil can be expected to work efficiently with widely varying rate of interruption. Thus, a coil wound to suit a low rate cannot be ‘ saturated’ by each fractional current sent to it by a break giving a much higher rate of interruption; and, conversely, a coil wound to suit a high rate of interruption cannot respond efficiently to the longer periods of excitation allowed by a more slowly acting break. The duration of each contact, or * make,’ during which current is allowed to pass to the coil is also determined by the interrupter, and may be varied according to the result desired. For this also the coil should be adapted. An important point in the action of an interrupter may be here noted—namely, the break of the current must be as sharp and sudden as it can possibly be made. Many workers have made the mistake of applying to the same coil interrupters of vary- ing construction and rate of interruption, and have been prone to judge the interrupter according to the result obtained, possibly ascribing failure to the construction of an interrupter rather than to the true reason of inco-ordination between it and the coil employed.

A coil may be badly damaged also by using with it a break different from that with which it was designed to work. Thus, by substituting a break which passed a much heavier current than the break originally used, we had a coil seriously injured, and we know of at least one London hospital where several coils were rendered useless by such a change injudiciously made.

The choice of coil and interrupter depends upon the demand likely to be made on them—that is, upon the nature of the work to be done.

Chapter 3
INTERMEDIATE APPARATUS
Interrupters or Breaks—Induction Coils—Valve Tubes, etc.
[Appended to this chapter will be found an explanation of the action of an induction-coil with vibrating break, and instructions for use, which we have by request dealt with in some detail.]

FOR excitation of an X-ray tube, we have seen to be required an electric current of very high potential or electro-motive force. No source other than a static machine directly supplies current of sufficiently high potential; therefore, unless in the case of such machine, a transformer must be interposed between the source and the X-ray tube. For this purpose an induction-coil is ordinarily used. But the current supplied to an induction-coil must be a regularly interrupted current, and to produce such interruption an auxiliary piece of apparatus is usually employed, known as the interrupter or break.

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.