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.

The rate of charging may be indicated while in process by an ammeter in the circuit, but usually the means of charging is arranged for a certain rate, which is thus fixed. The number of amperes permissible varies somewhat for different types of accumulator, but should always be in rela- tion to the capacity of the cells. The maker usually marks on each set as sent out the proper maximum rate of charge and discharge, and these should never be exceeded, or the life of the accumulator will be considerably shortened. As a general rule, the charging current, expressed in amperes, ought not to exceed one-fifth, and the discharging current one-fourth, of the number expressing the capacity in ampere hours; thus, a 20-ampere-hour accumulator should not be charged with a current of more than 4 amperes, nor discharge more than 5 amperes.

Before commencing to charge, see that the plates are well covered by the acid; and, if the cells be covered or sealed for transport, see that the blow-hole is left open during the process of charging.

With a knowledge of the rate and duration of discharge of the accumulator since last recharged, one may calculate the time required, with a known rate of charging, to restore the total ampere hours of its capacity. But little, if any, harm can be done by overcharging, and full time should always be allowed for completion of the process. A ready method of telling when a cell is fully charged is to listen for the hissing effervescing sound produced by escape of the free gases. The intensity of this will depend, of course, on the strength of the charging current, but a little experience soon detects the indication. The acid, at the same time, turns milky in appearance.

Frequent recharging tends to preserve the efficiency of accumulators, and they never work better than when used and recharged daily. Even when not in use, they should be occasionally recharged—at least once in every three weeks—to keep them in good order. If one goes off for a holiday, they should be fully charged before being left.

If left at rest in a charged condition for any length of time, accumulators tend to deteriorate (as explained later); but they do so much more rapidly if left standing discharged.

The number of plates varies, but in each cell of those we use (as in that shown in Fig. 18) there are seven plates, three positive and four negative. The positive plates are of a dark chocolate colour, while the negative are of a slaty-grey. The dilute sulphuric acid in which they are immersed should be of a specific gravity of 1190, attained by adding 1 part of pure H2S04 to 5 parts of water. The dilution should, when cold, be tested by a hydrometer supplied for the purpose, and should be adjusted, if necessary, by addition of more H2S04 or water according as the indicated density is below or above the desired standard.

The box encasing the cells, and its divisions, are usually lined with lead, and little strips or buttons of wood or glass are placed between the plates, to prevent them coming in contact during transport. If adjoining plates were allowed to touch each other while the cell was charged, they would be rapidly destroyed by the strong discharge of electricity. Over each cell of a portable set is a vulcanite or wooden cover, with a blow-hole in the centre; which hole must be plugged when the cells are being moved, but left open when the cell is being charged. Each box has its terminals painted, the positive red, and the negative black.

The capacity of an accumulator depends mainly and directly upon the quantity of lead in its plates, and is ex- pressed in ampere-hours. Thus, * 60-ampere hours ‘ signifies that an accumulator can discharge 1 ampere for sixty hours, 2 amperes for thirty hours, etc.

Charging.—In charging an accumulator, the wire from the positive pole of the source must be connected to the positive terminal—that painted red—and the negative wire to the negative terminal—painted black.

It is of prime importance that no mistake be made in this connecting; for, if wrongly done, the accumulator will rapidly discharge, and may be totally destroyed.

A ready method of judging the relative polarities of the two wires leading from any source of supply is to lay them, about i inch apart, on a piece of moistened litmus-paper. At the positive pole is produced a red colour (acid), and at the negative pole a blue colour (alkaline).

Accumulators are often spoken of, somewhat loosely, as ‘ storage cells’ of electricity. This they are in effect, for, after receiving an appropriate * charge’ of electricity, they may be kept for some time, and thereafter a ‘ discharge’ of electricity obtained from them; but the term is misleading.

More properly they might be termed * energy transformers,’ or storage cells of energy. An electrical current, passed through an accumulator cell, produces chemical changes in the constituents of the cell, which thereafter stores energy in a potential form, represented by the force impelling the con- stituents to return to their former condition.

An external circuit being completed between the terminals of the cell so charged, its constituents more or less gradually resume their former condition, and the potential energy thus liberated appears in the form of electrical effects in the circuit.

The chemical changes referred to are somewhat compli- cated, and we do not here attempt a full explanation.

Each cell (as shewn in Fig. 18) contains several lead plates, connected alternately to the positive and negative terminals of the cell; and all are immersed in, and well covered by, dilute sulphuric acid.

During charging, a current of electricity is sent through the cell from a suitable source of supply. By electrolytic action there are produced from the acid quantities of free oxygen and hydrogen. The former appears on the plates connected with the positive pole, converting them gradually into peroxide of lead; whilst the hydrogen appears on the negative plates, reducing them to a porous, spongy mass of metallic lead. Those actions on the lead plates having pro- ceeded as far as possible, the gases escape in bubbles from the liquid, thereby indicating the completion of the charging process. In discharging, those processes of oxidation and reduction are reversed; the plates return to their previous condition, and the acid regains its original strength.

Thus the process may be repeated any number of times in the same cells, if due precautions be observed.

Some of those precautions we will now discuss, before describing the methods of charging accumulators from various sources of supply.

Accumulators for X-ray work are generally arranged with four cells in each box, as in Fig. 18. The cells are connected in series—that is, the negative pole of one is connected to the positive pole of the cell adjoining it, leaving at one end of the box a free positive terminal, and at the other a free negative terminal. The E.M.F. of each cell being fully 2 volts, such a box of four cells will give, when fully charged, at least 8 volts. Three such boxes are as a rule employed, thus obtaining an E.M.F. of 24 volts for use.

2. In the absence of a convenient direct supply for a permanent installation, accumulators may be used after being charged elsewhere; but if much work is to be done, it is better, in our opinion, to instal some form of generating apparatus to provide a direct supply. This may be understood when we come to speak of the process of recharging accumulators, for, if a primary source be inconvenient, then the chances are that the accumulators will not be recharged so frequently or regularly as they ought to be in order to preserve their efficiency. Under such circumstances, also, transport of the accumulators renders this a troublesome and costly method for regular working. Here again, however, the use of accu- mulators may be combined with the installation of a primary source of supply, where that may be more conveniently or economically brought into action only at periodic times.

8. Where a supply of any nature is available inter- mittently, accumulators are eminently serviceable to render the energy available as it may be required.

For conversion of an unsuitable regular supply accumu- lators may prove of service, as in the method, for a time employed by us to utilise an alternating current, described on p. 44.

In most cases we think some of the other means suggested will prove more commendable, but each case must be con- sidered in relation to its own special conditions.

Usually, where the use of accumulators is indicated, it will be necessary for the X-ray worker to superintend their re- charging, and to see personally that they are maintained in a state of efficiency. This personal responsibility may be obviated if the cells be sent to an electrician, or to a generating- station, where expert attention may be expected; but such, as we have said, is a most inconvenient and costly mode of working. Further, it is very frequently under circumstances which render it impossible to obtain expert assistance that accumulators will be found most useful.

The operator being thus directly responsible, we have, on request, decided to enter more fully into the questions of charging and working accumulators than into most of the other details dealt with in this small work. For men in army or navy service we trust this will be especially useful.

(d) Electrolytic interrupters are said by some to work satisfactorily with alternating current, preferably with an aluminium cell in series. The nature and use of this type of interrupter is described later on p. 58, and we highly commend it under suitable conditions; but we do not consider an alternating main one of those con- ditions.

(e) A number of other so-called «rectifiers’ have been designed to render an alternating current unidirectional, but in our experience none of those have proved of practical value.

II. Accumulators.

Accumulators are valuable as sources of direct supply—

(1) Where portability is of prime importance;

(2) Where there is an existent source of supply, but not convenient for direct connection by wiring to the X-ray installation;

(3) In occasional cases where supply is available, but not of a nature for direct use.

1. Where an X-ray outfit is chosen mainly with a view to portability, the advantage of accumulators will depend largely upon convenient opportunity of recharging them, since they can by no means be made to produce electricity unless in the degree to which that has been previously supplied to them.

Elsewhere in this chapter (pp. 46 and 47) we suggest possible arrangements of a portable nature where the set must be self- sufficient for all purposes. With such apparatus for primary production of electrical energy, it will probably be advan- tageous in many cases to combine the use of accumulators. These may be periodically charged from the primary source, and kept in readiness for immediate use at any time; whereas it may be inconvenient to bring the generator into action when current is required.

For bedside work accumulators are ordinarily the onty source of supply worthy of consideration, though the advent of a portable form of static machine would alter the definite character of this statement. The method of charging accu- mulators for such use will depend upon local conditions of the operator.

(c) Nodon’s valve, or aluminium cell, is a simple and valuable device for utilizing alternating currents for medical purposes, since it has no moving parts and requires little attention.

Such a cell consists of an active electrode of aluminium, immersed in a suitable solution (in some of common salt, in some of ammonium phosphate), along with an indifferent electrode of lead, carbon, or iron; the containing cell in many designs supplying the latter. The action of the cell depends upon the peculiarity of aluminium that as kathode it allows a current to pass freely, but as anode it offers high resistance. This resistance is due, doubtless, to polarisation, whereby a thin, insulating film of oxide is formed on the surface of the metal. The other phase or direction of current easily breaks through this, but up to a certain voltage the cell automatically allows only that phase to pass.

By suitable combination of cells, explained later, both phases may be utilised, and a pulsating unidirectional current obtained from an alternating supply.

Heating of the liquid will interfere with the efficiency of a cell if the temperature rises above 70° F.; but short of that those cells may do a lot of heavy work, especially if arranged as advised on p. 48, and shown in diagram there.

For X-ray work the current produced is not satisfactory, either led direct to the coil or passed through an interrupter.

For charging accumulators from an alternating main, however, these cells work very satisfactorily. In the note on accumulators already referred to will be found a description of the arrangement.

In our hospital work this was the original plan adopted, and it served fairly well for about two years, until pressure of work induced us to discard it for the less troublesome plan of a motor-transformer.

Fig. 17 represents a set of cells as manufactured for sale, but efficient cells may be made very cheaply by anyone willing to take a little trouble. For description of such a cell and other practical points, see p. 42.

A motor-transformer is extremely easy of manipulation, and, beyond ordinary care of lubrication and attention to brushes of the generator, requires no attention, expert or otherwise.

The starting-switch is usually made of a form to send the driving current into the motor gradually, since the full current, suddenly switched on to the motor winding, might readily do serious damage to it. In ordering this piece of apparatus, the voltage of the supply must be noted, as well as the nature of the current desired to be generated for use.

2. Alternating current is unsuitable for direct use, and must of necessity pass through some modifying apparatus before reaching the X-ray installation.

(a) A motor transformer is, in our opinion, the most satisfactory means of adaptation, the motor being wound to drive off the alternating circuit, and being coupled direct to a dynamo designed to supply continuous current as desired. In the West London Hospital the current supplied from the main is alternating, at 110 volts, with a periodicity of 50 per second. This is led to a 2J horse-power motor-transformer, from which continuous current is derived at 50 volts, with a maxi- mum of 20 amperes, for use in the X-ray depart- ment.

The above note and figure on motor-transformers for converting the voltage of continuous current from the main apply equally to those designed for converting alternating current. In this case it is necessary, in ordering, to state the voltage of supply, its period, and its phase—single, triphase, etc.

(b) The Gaiffe-Blondel mercury jet break, shewn in Fig. 16, is an example of a type of machine designed to work synchronously with the period of an alternating current, and so transmit the impulses in one direction, while arresting, or diverting, those in the other. This action produces an interrupted unidirectional current suitable for supply to an induction-coil. But, in our experience, though this arrangement is good for high- frequency or for therapeutic work, the synchronicity may occasionally fail. Thereby the polarity of the resultant current may become reversed, and the irregular rays thus produced in an X-ray tube spoil an exposure for radiographic purposes. Along with a Villard’s valve- tube to protect the X-ray tube from such inverse currents this break may, however, form a satisfactory means of utilizing an alternating current.

1. Continuous current from the main, where available, should certainly be used direct for any permanent installation. If at a voltage of 100 or less, the current may be sent unaltered to a suitably-designed interrupter and coil. If at a higher voltage (as in West London, where voltage is 240), some form of modifying apparatus must be interposed. In this country the usual supply from the mains is continuous, at a pressure of 200 to 250 volts.

(a) A rheostat in some form may be introduced, whereby current at any voltage in a regular series may be used as desired. The usual form consists of a number of open coils of thick iron wire, suitably fixed to a slab of slate or marble, and connected, as shewn in Fig. 18, so that any length may be introduced as resistance in the circuit. This piece of apparatus, from its function, is often termed a * volt selector.’ Its principle may be understood from the annexed diagrams (Figs 13 and 14). In « direct’ resistances the current is directly ‘ choked off’ before reaching the further parts of the installation, as in the smaller coil shown in Fig. 14. In a ‘ shunt * arrangement, as shown in Fig. 13, and in the larger coil of Fig. 14, an alternative path is offered to the current, and more or less of the available current passes by this alternative circuit as more or less resistance is intro- duced into the installation circuit, which receives current inversely proportional to the resistance. This latter form is preferable, as the transition is rendered more smooth and gradual between adjoining steps of the selector. In both instances the residue of current disappears in the resistance, being spent in the production of heat in its coils. Thus such an arrangement may appear wasteful of current; but it is really more economical than an individual supply for the relatively small quantities usually required, and current is supplied from the main for such purposes at reduced rates.

Fig. 14 shows diagrammatically a very convenient arrangement, whereby a high voltage (say 250) is first adapted by a shunt resistance to 100 volts, then current selected as desired from a series resistance giving a range from 100 to 50 volts.

(b) A motor-transformer is probably more economical where larger quantities of current are likely to be used, as in regular hospital work. This piece of apparatus, as shown in Fig. 15, consists of a motor constructed to run at the voltage of the main supply, and connected direct to a dynamo wound to give off current at the voltage desired for use. In our opinion, 50 volts is the ideal strength for general X-ray work, since it is sufficient for all practical purposes; and disturbance in the induction-coil is obviated from the inverse currents, which at higher voltages become of sufficient magnitude to be troublesome.

Chatper 2

Source of Supply

THE source of supply of electricity for X-ray work may be one of a number of varying description.

The work to be done will dictate the choice where that is an open one; but more often the choice will be dictated by the factors of convenience and economy relative to the installation under consideration.

Undoubtedly the best method presently available for all- round work in this country is to employ a continuous current of 50 to” 100 volts, and to convert that to the high potential necessary to operate an X-ray tube by passing it through an induction-coil.

We are aware that in America the static machine is the favourite source of supply, and we believe that with some alterations in construction, likely to be introduced at an early date, that machine will find increasing favour in this country, where climatic and other conditions render its use somewhat uncertain in its present forms. The tendency of modern workers to employ softer tubes will also encourage this

Where a supply already exists in some way different from that here recommended, we believe that it is always most satisfactory to convert it into that form.

This may mean initial expense, but, where any serious amount of work is to be done, that will soon be compensated for in convenience and economy of working.

The various sources of supply ordinarily available are: change.

I. The main. This is the most satisfactory source, and may be (1) continuous, (2) alternating.

II. Accumulators.

III. Primary batteries.

IV. Dynamo machine.

V. Static machine.