Thus, the nature of the secondary winding must be decided  according to the balance of those two factors deemed most  suitable for the work to be undertaken. The relation is not  really so simple as here stated, other factors complicating it;  but discussion of those is not essential to a working under-  standing of the coil. Beference to the previous discussion of  the most suitable current for exciting X-ray tubes (on p. 63)  will explain the interest and importance of this point.

2. Strength of the primary current is, of course, under  our direct control. Use of very heavy currents compels con-  sideration of special points concerning self-induction of the  primary winding, but those do not come within the purpose of  this section, since such currents are prohibited by the use of  a vibrating break.

With such a break it is said that a current of much more  than 20 volts cannot well be used, since the platinum contacts  will wear away too quickly and will have a tendency to stick,  thus endangering the primary winding.

We have had good results with a current of 24 volts, and,  with careful working, had little or no trouble; but probably  we were very near the margin of the safe limit.

Thus, in the secondary circuit is set up a series of currents  alternating in direction, their periodicity depending upon the  rate of interruption of the primary current.  The inductive effect is augmented by the presence of the  iron core, with its alternate magnetisations and demagnetisa-  tions, which are in effect similar to rapid movements of a  strong magnet alternately toward and away from the secondary  windings.     The E.M.F. or voltage of the secondary currents will  depend upon and vary directly as—

1. The number of turns of wire in the coil;

2. The strength of the primary current; and

3. The suddenness of the break of the primary.

The strength, quantity, or amperagre, following Ohm’s  law, will vary directly with those factors, and inversely as  the resistance of the secondary circuit.

1. A very long secondary winding  high E.M.F., with discharge sparks of great length; but its  concomitant high resistance will prevent a great quantity of  current passing, and the sparks will be correspondingly thin  and thready.

We will not describe details of construction, since those  vary somewhat, and are dealt with in a more or less lucid way  to suit the resistance of different X-ray tubes than does a  valve-tube.

By screwing the point home till it touches the plate the  spark-gap may be abolished; by screwing back the point the  resistance can be increased gradually, till the light in the  tube indicates that the inverse current has ceased.  Where tubes of different hardness are used in succession,  this ready means of regulation is of great value, and its effect  on the appearance of a tube is often very striking.  in most catalogues of electro-medical instruments, as well as  in works on theoretical electricity.  The external appearance, doubtless familiar to all our  readers, is recalled by Figs. 29, 87, and 45.  Fig. 41 shews diagrammatically the arrangement of  essential parts.

Considering the primary current as entering at (A +), it may  be traced up the metal pillar (G), across platinum points at  (H) to the hammer (J), down the spring (D), and thence to the  primary winding of the coil (bb). By that it is led round the  core (aa), then back to the other terminal (A-).  The core (aa), consisting of a bundle of wires or thin sheets  of soft iron, becomes rapidly magnetised by influence of the  current circulating round it.

An adjustable spark-gap, as illustrated in Fig. 39, may  be interposed in the secondary circuit for the same purpose of  cutting out inverse currents, and is so used commonly with  static machines. For heavier currents, as from coils, this is  a somewhat noisy working arrangement; but such an arrange-  ment as is shewn in Fig. 39 allows much more easy regulation  and the valve-tube is interposed on that side. Fig. 38 shews  the arrangement diagrammatically, but in practice the valve-  tube should be interposed, as in Fig. 40, between the coil and  the sparking pillars, otherwise the alternative spark will  measure the resistance of the valve-tube in addition to that of  the X-ray tube, and thereby convey a false idea of the condition of the latter.  Readings on a milliamperemeter placed in the secondary  circuit show that this action is more than theoretical, and

The use of the spintermeter and milliamperemeter in  the secondary circuit of the coil has already been explained,  and the annexed diagram (Fig. 40) shews their arrangement  during operation.

Hammer Break.

Since the efficient working of an induction-coil depends in  so large a degree upon an intelligent understanding of its  principle and construction, we have on request decided to add  here, as in the case of accumulators, some more theoretical  and detailed instructions to workers.

A tube under such conditions is not reliable for photo-  graphic exposures, and rapidly suffers in quality in the  manner described in the section on ‘Changes during To check off these inverse currents some arrangement is  frequently interposed in the secondary circuit between the  coil and the anode of the X-ray tube, and for the best photo-  graphic effect this should always be done. Such check is  necessary with high voltage, with a rectified alternating  current, or with a very rapid series of interruptions.

A Villard’s valve-tube, or ‘ soupape/ is the usual piece of  apparatus so employed. This consists, as shown in Fig. 38,  of a vacuum-tube of moderate degree of exhaustion, having  one end drawn out as a slender prolongation of the central  space. Into the main space projects a terminal of thick  aluminium wire in the form of a corkscrew; and in the  farthest part of the prolongation is the second terminal,  formed by a slender rod of aluminium. So long as the  larger corkscrew-shaped terminal acts as a kathode the tube  conducts easily, but to currents in the opposite direction it  offers a high resistance. If this tube be placed in proper  relation to the X-ray tube, it will be readily seen how it will  oppose the passage of the inverse currents described, whilst  allowing easy passage to the direct currents desired for use.  In series with an X-ray tube the correct setting may be  remembered by noting that platinum alternates with  aluminium. The platinum anode of the X-ray tube should  of course be towards the positive pole of the induction-coil,  experience in working installations also testifies to the  advantage of the device. A vacuum regulator should always  be attached to the valve-tube, otherwise needless resistance  may be opposed to the current.

The current sent from an induction-coil through an X-ray  tube consists of a rapidly recurring series of brief currents  induced in the secondary circuit in consonance with the  interruptions of the current sent to the coil from the inter-  rupter or break. This secondary current will depend initially  upon the strength of the primary current employed; further,  upon the rate of interruptions of that current; and, as regards  the coil, mainly upon the relative lengths of the primary and  secondary windings thereon. It may be recalled that this  inductive effect is due to the interruptions of the primary  current. The suddenness of those individual interruptions,  as well as the duration of actual passage of current in the  intervals, will affect the character of the secondary current  induced. As already noted, the break in the current produced  by the interrupter must be as sharp and sudden as it can  possibly be made.

At each ‘make’ of the primary current a momentary  current is induced in the secondary circuit in a direction  opposite or ‘ inverse’ to that in the primary, and at each  ‘ break’ there is induced a momentary ‘ direct’ current of  greater power. Those direct currents at break are alone  desired in the discharge of the coil for X-ray purposes, the  inverse currents, as mentioned earlier, being of damaging  effect. The actual E.M.F.’s of the two induced currents are  equal; but the current at ‘ make ‘ is more slowly induced, and  this delay is increased by nse of a condenser, while at’ break’  the secondary current is induced much more sharply. Thus  the currents at break may be said to be in quality more  ‘ impetuous/ and manifest themselves as sparks; while those  at make are more ‘ deliberate,’ and fail to form sparks under  ordinary circumstances. Where the potential of the primary  current exceeds 50 volts, however, the effect of these ‘ make’  or inverse currents becomes noticeable in the fluorescence of  the X-ray tube. This effect is marked by a flickering, greenish  fluorescence in the hemisphere of the tube ordinarily free  from illumination.

With 8 amperes passing, the break works on steadily with  a regular ‘ dick-dick’ bruit, but on approaching 12 amperes  the sound becomes lower-pitched and laboured, denoting that  the coil cannot absorb or utilise the surge of current supplied  to it.

We have said that a great potential spark-length is not  essential for a good coil for X-ray work; it is, indeed, un-  desirable. From a coil emitting very long sparks it is difficult  to obtain more than 1 milliampere of current, whereas with  the shorter and coarser winding suitable for shorter sparks  we may obtain a current of 10 to 15 milliamperes. No tube  presently made could stand that current for more than a few  seconds, so that alteration of coils in that direction is for the  present limited in its utility. Long sparks are correspondingly  ‘ thin’ and thready. What is now wanted for general use is  a spark of moderate length, but ‘ fatter.

Some recent coils have their primary windings made in  separate sections, so that a greater or less length of wire may  be put in circuit according to the strength of current supplied  to the coil, a shorter length, as represented by a less number  of sections, being employed for heavier currents.

Convert the E.M.F. of the current supplied to a voltage suit-  able for the X-ray tube. The degree of this function may be  appreciated when we note that to produce a 12-inch spark  across the discharging points of a coil requires a potential of  about 148,000 volts. This increase in voltage is obtained at a  corresponding loss in amperage, and the product of the two  factors—technically expressed as Watts—in the current  derived from the coil should approach that in the current  supplied, a slight loss being inevitable. No satisfactory  method exists, however, whereby the output of a coil may be  accurately measured.

The action of a coil is commonly expressed in terms of the  length of spark which it is able to send across the terminals  of its secondary when the primary is supplied with a suitable  exciting current, but this expression is misleading.  Formerly, indeed, it was the custom to consider the spark-  length as synonymous with the power of a coil to do good  X-ray work, and a spark of from 16 to 20 inches was  considered a desideratum for a good X-ray coil. But we now  recognise that a coil with a maximum spark of 10 to 12 inches  may be capable of satisfying all our requirements, and we pay  more attention to the nature or ‘ thickness’ of the spark  emitted. Thus, in our hospital installation, the coil gives us  the full spark-length with a current of 4 amperes, but we  commonly use 6 to 8 amperes, and may pass as much as  12 amperes at an odd time. The additional current does not  lengthen the spark, but increases its intensity or fullness.  This coil, we may say, is designed to work nominally with  interruptions at the rate of 600 per minute, this being  considered a mean rate for usual work. There is, of course,  a margin of reasonable efficiency above and below this rate.

It is important that an electrolytic interrupter should be  connected correctly, the platinum to the positive pole of the  source. If connected otherwise, the platinum will gradually  Induction-coils have been already referred to in the intro-  duction to this chapter, and some of the conditions of their  working discussed. We do not propose to enter into any  detail of their theory or construction, since those will be found  sufficiently described in any modern text-book on electricity.  (A few notes on their practical working are appended to this  chapter for the convenience of workers not familiar with  them.) The coil forms, however, a most important part of  an X-ray installation, and it is advisable that every operator  should make himself thoroughly acquainted with its theory  and practical working. For our present purpose it is sufficient  to recall that its function, broadly speaking, is to raise or  dissolve, or possibly fuse if thin, and the coil will not work  well. If the direction of current be correct, the sparks in the  interrupter have a red colour; if wrong, they have a blue  colour. Litmus-paper can, of course, give us the necessary  indication before connection is made, the positive pole making  a red stain on the moist paper, as described in the section on  the charging of accumulators.

On the Continent electrolytic interrupters are used com-  monly, but in conjunction with coils specially made to suit  them. The chief check to their use here is the heavy  mortality amongst tubes, most of which can stand the heavy  current transmitted for a very brief period.

In action, and especially with heavy currents of high  pressure, electrolytic breaks are very noisy; so the whole cell  and attachments should be cased in felt, to deaden the sound,  and the break should be kept in a separate closed room where  possible.

With heavy work the electrolyte becomes very soon heated,  and operation of the break is thereby embarrassed, and later  stopped. A cell of large capacity should therefore be employed  to delay the heating effect, and that may be set into a larger  vessel containing cold water if continuous heavy work is  expected.

The cells require no cleaning, which is a great convenience.  For use with currents of small quantity and low potential,  as from batteries, these breaks are unsuitable.  They require for efficient working a current of about  40 volts or more, will not work under 30, and work best  between 60 and 80 volts. Unless for specially strong currents  indeed, such a break is not advisable, since its action is not  reliable enough to commend it for ordinary purposes for  which other breaks may serve.