Computed Tomography

Chapter 3 Part 21

Tom Thym — Mon, 21 Dec 2009 17:13:51 +0000

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.

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Chapter 3 Part 20

Tom Thym — Sun, 20 Dec 2009 17:12:47 +0000

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.

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Chapter 3 Part 18

Tom Thym — Fri, 18 Dec 2009 17:11:39 +0000

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.

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Chapter 3 Part 17

Tom Thym — Thu, 17 Dec 2009 17:11:02 +0000

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.

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Chapter 3 Part 16

Tom Thym — Mon, 14 Dec 2009 17:10:17 +0000

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.

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Chapter 3 Part 16

Tom Thym — Sun, 13 Dec 2009 17:09:34 +0000

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.

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Chapter 3 Part 15

Tom Thym — Sat, 12 Dec 2009 17:08:56 +0000

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.

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Chapter 3 Part 14

Tom Thym — Fri, 11 Dec 2009 17:08:14 +0000

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.

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Chapter 3 Part 13

Tom Thym — Thu, 10 Dec 2009 17:07:30 +0000

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.

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Chapter 3 Part 12

Tom Thym — Wed, 09 Dec 2009 17:06:54 +0000

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.

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