A break so constructed we know from experience to work  very satisfactorily, and it can be renewed many times for the  cost of one of the more elaborate breaks put on the market,  which will give little, if any. better results.

With electrolytic breaks the frequency of interruption will  vary according to the pressure of current supplied, higher  voltages producing more rapid interruption. By exposing a  larger area of the platinum anode to the fluid the rate of  interruption is reduced, and by exposing less the rate is  accelerated. The increased area of exposure allows a higher  amperage to pass. With the electrodes suspended from  separate strips of wood, glass, or other insulating material,  as shown in Fig. 36, some regulation of the action may be  secured by moving the two electrodes to various distances  apart.

For working with this form of break, an induction-coil with  short primary winding should be employed, since the self-  induction of longer primaries prevents the full effect of the  break’s special efficiency reaching the X-ray tube.  The condenser of the coil should in all cases with these  breaks be put out of circuit.

An arrangement, which constitutes probably the utmost  extent of elaboration consistent with efficiency and economy  in regular work, might consist of a large-sized glass cell,  having immersed in the fluid a cylinder of porcelain, from  the bottom end of which a platinum wire projects to an extent  variable by a screw at its upper end, and having, further,  a kathode in the form of a sheet of lead of suitable size.  The triple type illustrated in Fig. 85 is now generally used,  as it permits a much wider range of adjustment and regulation  (in combination with the primary winding of the induction  coil) than the single type just described.  As further illustrating the simplicity in essentials of this  form of break, we represent in Fig. 86, and describe, one  which may be made for himself by anyone who has even but  slight manual dexterity, and that for a few pence, or, at most,  a few shillings.

An ordinary earthenware or glass jar may be used as a  containing-cell—the larger the better. Through the closed  end of a test-tube seal a short length of platinum wire, and  into the tube pour mercury to a height of 1 inch or more.  Through a wooden cover to the cell, or in some other suitable  manner, as in Fig. 36, suspend the test-tube immersed deeply  in the fluid, and to make connection, dip a wire from the A, Glass jar containing dilute sulphuric acid (1 in 10); B, sheet-lead with  terminal; C, test-tube with platinum point and containing mercury.  positive pole of the source of supply into the mercury. Then  hang over the edge of the cell a piece of sheet-lead reaching to  near the bottom of the fluid, and having a screw at its upper  end to bind a connecting-wire from the negative pole of the  source, and your electrolytic break is complete.

3. Electrolytic breaks, commonly known as Wehnelt’s  breaks, from the name of their originator, are now extensively  used as specially suitable for the heavier currents desirable for  rapid radiographic work. With these breaks a much more  rapid rate of interruption can be obtained, and they are  capable of transmitting currents heavier than any X-ray tube  at present made can stand for more than a few seconds.  They may be used with alternating current (as mentioned  earlier) or with continuous current, but work more efficiently  with the latter.

As the name indicates, these breaks depend upon the  electrolytic action of a current passing between electrodes  immersed in a liquid. If one electrode be of very small area,  the bubbles of gas formed tend to collect on it, and thus  interrupt the continuous passage of the current; then almost  instantaneously these are dissipated, and the current is again  free to pass. A regular succession of accumulation and dissi-  pation of bubbles renders in this manner the current passed  through the cell intermittent in character, and the periodicity  so obtained is much more rapid than with any form of  mechanical interrupter in use. In practice, the electrode of  small area is the anode, and is composed, of platinum, while  the kathode consists of a lead plate of large area.  Dilute sulphuric acid (1 in 10) is commonly used, though  other fluids have been suggested as more suitable for certain  purposes. A cell containing such fluid, and having immersed  in it two electrodes as described, represents the total essentials  of the break, though many elaborations of adjustment have  been introduced.

In a recent modification called ‘ The Intensive Break’ very  heavy currents, quoted as 25 amperes at 110 volts, may be  passed through such a break if desirable. This is a decided  advantage, though the price is somewhat higher than for the  ordinary form.

A recent London make (illustrated in Fig. 34) is driven  by a star-shaped magnet of soft iron, which is mounted on  the upper end of the jot shaft. This is rotated by influence  of the successive magnetisations and demagnetisations of the  core of the induction-coil, opposite the end of which the in-  strument is set. In this break the jet-producing portion is  enclosed in an air-tight metal reservoir filled with coal-gas or  hydrogen, which acts as a dielectric to cover the points of  contact instead of the usual liquid. This avoids the forma-  tion of sludge, and consequent inconvenience of frequent  cleaning. The speed may be regulated somewhat by altering  the position of the interrupter relative to the coil, thus re-  quiring no special rheostat; but this regulation of speed is  not thoroughly satisfactory. The break is quiet in action,  even with heavy currents, is of comparatively moderate price,  and we reckon it one of the best turbine breaks on the  market.

To avoid the starting by hand, which necessity may be  somewhat inconvenient, the makers also supply a small  independent coil, by which the break may be driven and  continue in rotation whether the larger coil be in or out  of circuit.

With this type of break at rest, there is no danger of the  continuous current being accidentally passed direct to the  coil, since the jet by which the contact is made is not formed  until some rate of speed is got up.

A higher rate of interruption may be obtained with such^a  break than with a dipper; but for lower rates of interruption  those breaks are not good. For a time the action at a suitable  rate is very satisfactory; but the break is somewhat easily  put out of order, and requires frequent cleaning, which is a  difficult and dirty process. More recent forms are simpler in  construction and more easy to clean when clogged.

The earlier forms (as in Fig. 32) are driven by belt-con-  nection from the motor, and the slipping of this may cause  much inconvenience. More recent jet breaks are driven by  some electro-magnetic arrangement directly connected to the  rotating parts.  Thus, in Gaiffe’s form (as shewn in Fig. 33), four electromagnets are situated on the cover of the instrument, and  through the coils of these the current passes before entering  the interrupting mechanism on its way to the primary of the  induction-coil. This is a doubtful advantage, since the rate  of interruption cannot be varied independent of the strength  of current supplied to the coil, or vice versa; whereas that is  a latitude desirable under certain conditions. With a heavy  load the action is somewhat noisy ; but this is on the whole a  reliable instrument, and has the further advantage of being  less than half the price of the older forms.

When at rest, the plunger or vane of a dipper break may  be arrested at the end of its dip—that is, immersed in the  mercury—and if the current were suddenly switched on, it would  recovery much less unpleasant. Methylated spirit is com-  monly used, but it is better to use rectified spirit; for,  though initially more expensive, it requires changing much  less frequently, and, further, has less corrosive action on the  metal parts. The level of the alcoholic liquid must be kept  well above the highest point of travel of the dipper, or other-  wise the liquid may ignite, causing an alarming, if not  dangerous, explosion.

To clean, the emulsified liquids should be allowed to settle,  the clearer spirit settling out on top decanted off, and the  mercury heated gently in a retort with condenser attached,  pass direct to the primary of the coil, and possibly do much  damage. Hence, it is important to see always that the motor  is in action before the supply current is switched on. This is  secured in our installation, as described later, by a special  form of switch.

The churning action of the break gradually causes some  degree of emulsification of the mercury and the covering  fluid (or dielectric); thus these materials require periodical  renewal. More or less of the mercury can always be recovered,  and, with this in view, alcohol will be found to be the prefer-  able material for a dielectric, as making the process of  whereby the remaining spirit is vaporised and the mercury  recovered for further use.

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.