Oscar N. Dame

In the vibrating form of interrupter, whether core actuated or not, platinum is used for contact points because of its conductivity, hardness and freedom from smut or oxidization. The nearest approach to platinum for this purpose is coin silver, which is a slightly better conductor when oxidized than ordinarily, and much cheaper, but does not give lasting satisfaction, and is therefore used only on the simplest apparatus.

Iridium in combination with platinum makes an deal contact metal. These points are extremely hard and are cast in " drops " of convenient size and ground flat when needed. For battery coils of the highest types, all vibrators are equipped with these points.

The amateur will readily understand that these points have to be flat at the point of contact so that the greatest amount of current may reach the primary of the core, for upon liberal saturation of the primary depends the generation of powerful lines of force cutting the secondary turns. Yet, on the other hand, on a vibrating spring only a few inches long there is a. limit to the contact area of the pieces of platinum. Most interrupters have points of about No. 42 wire gauge.

Fig. 1.

Condensers and their operation and construction have been so ably described in past issues of Amateur Work that I will pass this necessity in vibrator design with only a suggestion to amateurs about to construct one for home use. As is well known, a condenser to meet all conditions of coil operation must be varied in capacity to meet the conditions of the primary and battery used, also the frequency of vibration. It is advisable to make a number of small condensers which may be connected together in multiple, series or compound to give certain capacities needed. Amateurs will understand that the condenser is built up to the proper capacity to cut the contact spark to a minimum, and that the addition of more capacity will prove too much for the coil, thereby weakening and shortening the length of the secondary discharge. It will be seen, therefore, that there must be some sparking at the make or break points, or else the secondary discharge will fall short of maximum, and this condition of affairs furnishes plenty of opportunity for experimental condenser construction.

Fig 2.

Then, again, it will be observed that when the secondary spark balls are separated the maximum limit, the sparking at the contact points will be heavier than when the same coil is operated with the same primary battery and the spark balls closed to a fraction of an inch.

The vibrating interrupter admirably serves its purpose for small coil operation, and the cost of construction will not be great. But the amateur soon discovers that there are superior methods of interruption which bring out more of the spark value of the coil at but little additional expense.

First among other types of interrupters is the rotary, illustrated in Fig. 1. A large commutator is rotated between two carbon or copper flexible brushes, and as the insulated part of the commuter meets and passes the brushes, an interruption of the primary current takes place. Often times this commutator and brushes are immersed in alcohol or kerosene, which prevents sparking at the make and break. Condensers are used with this type, to absorb the extra self-induced primary current, as in styles previously mentioned.

Early experimenters in coil work found mercury a very appropriate medium for interrupting electric currents. Being liquid and an excellent conductor, an iron needle or rod could be immersed and removed up to 1000 times a minute by means of the simplest of mechanism, and with motor attachment to increase the speed, interruptions as high as 3500 were obtained. Alcohol or thin oil in the mercury jar, prevented undue spattering and confined the spark at the make and break.

Another type consisted of a disc of steel, rotated at high speed and the disc being cut away at one edge for a short distance made an interruption every revolution. Many interrupters of this type are in service today in England, where it was first introduced.

Fig. 8.

A third type of interrupter is based on the principles of the Archimedian screw. Turn to the pages in Deschanel's or Avery's Physics devoted to the Archi-median screw and note how readily this device may be installed for interrupter work. Rotation of the screw lifts the liquid from the bottom, up through the spiral turns to the top, where it falls to the trough again to be lifted. Mercury passes through the spiral as readily as water. It may cause the interruption by being dropped or forced upon a vane or blade of metal, or may complete a circuit by dashing its discharge against the mercury issuing from another pump. This seems to be the most feasible type of mercury interrupter in use today. In the X-ray laboratory, especially where tubes are being exhausted, it plays an important part. These mercury interrupters are used with all voltages up to 110 with little perceptible heating.

A great many operators now use the Wehnelt break with results which are extremely satisfactory. Owing to the large number of breaks (1000 or 10,000 or more per minute) the amount of energy delivered by the Secondary is greatly in excess of that possible with any other interrupter save the Caldwell form or a high speed mercury device; in making Skiagraphs, therefore, the time of exposure should be greatly diminished assuming that, other things being equal, the X-rays are proportional to the energy output per unit of time. That this is the case is proved by the results which have come to us. X-ray pictures through the body formerly requiring 15 to 36 minutes are now secured with exposures of from two to five minutes. Flur-oroscopic images do not depend upon the energy output so much as upon the quality of the ray, and hence are not necessarily better with an Electrolytic than with an ordinary break except that absolute evenness and steadiness is secured with the former, whereas with the latter, unless the rate of interruption is high, there is flickering.

A Wehnelt interrupter consists essentially of a small surface (3 to 4 sq. millimeters) platinum anode and a large surface (200 or 300 sq. centimeters) lead kathode immersed in dilute H2S04. If joined in series with the primary of an induction coil and sufficient E. M. F. exceedingly rapid breaks take place at the platinum surface. These breaks are probably due to the sudden formation of an envelope of nonconducting gas about the platinum surface; their frequency varies directly as the E. M. F. employed and inversely as the area of platinum. In practice at least 40 volts at the terminals of the interrupter must be used to obtain good results; as on large coils from one to four amperes will be required, this causes a heating loss of 40 to 160 watts so that the interrupter is theoretically not efficient. The rate of interruption is also seriously affected by the heating of the dilute acid; if heating goes too far, so that the fluid becomes actually warm to the touch the interruptions seem to lose their sharpness and will often fail altogether. This shows the necessity of keeping the fluid at a uniform and reasonably low temperature.

When using a Wehnelt interrupter no condenser is required; this reduces the cost of a coil somewhat.

As the interruptions lose all sharpness when lead is made the anode and platinum the kathode this form of break permits the running of induction coils on alternating circuits, only positive impulses giving secondary discharges. There is, however, considerable corrosion of the platinum and its holder on the negative impulse thus making maintenance expensive, and it is, therefore, not advised for alternating use except when no other method of running is possible.

Phospher bronze is an alloy of phospher, tin and copper, containing usually 0.053 to 0.76 per cent phosphorus and four to ten per cent tin, balance copper. It is as tough as wrought iron, more ductile than copper and is capable of withstanding great wear.