X-Rays, Apparatus and Methods Page 9

This is on account of the independence of the coil and the break, thus permitting the breaking of any desired value of current without the slightest change in either frequency or character of break. With the usual vibrating break, oper- ated by the core of the coil, the character and frequency of break is necessarily a function of the value of current, being broken and changed with it. It is impossible, with a large coil, e. g., to get uniform and continuous secondary discharges of a length very small compared with the maximum capacity of the coil, since to such short sparks would corre- spond a magnetization of the core insufficient to operate the vibrator at all. But with the rotary break we may make the current as small as we please, without in the slightest degree interfering with the frequency and precision of the break.

Two milled heads at top of can permit its lid (carrying brushes) to be withdrawn. A similar head at top of shaft loosens break-wheel, and two heads at bottom of can allow it to be lifted away for cleaning and renewal and replenishing of water.

We use three makes and three breaks per revolution and find best results at from 1,200 to 1,400 revolutions per minute (equal to 3,600 to 4,800 interruptions). We may secure as many as 2,000 revolutions (6,000 interruptions) per minute, by cutting out the motor regulator.

In operation, the brush bearing upon break-wheel should be positive, so as to prevent any electrolysis of the break-wheel. We have often broken between 15 and 20 ampères through this wheel for a considerable length of time with- out any overheating or excessive wear of the wheel. Should the wheel wear in time to an undesirable degree it may be removed and turned down. Provision is also made for replacing the brushes when desired.

General Considerations Regarding Break Frequency. — The question as to whether there is a certain best frequency of break for a given coil or tube or both, has puzzled not a few. Very little has been experimentally determined regarding this point. It is certain, however, that, with a given coil or tube, shortness of exposure is not exactly inversely as the