X-rays and localisation / by James Mackenzie Davidson.
- Davidson, James MacKenzie.
- Date:
- [1902]
Licence: In copyright
Credit: X-rays and localisation / by James Mackenzie Davidson. Source: Wellcome Collection.
Provider: This material has been provided by UCL Library Services. The original may be consulted at UCL (University College London)
2/12 (page 2)
![[April 25, in the primary, is about fifty times more powerful than the current produced at the make in the primary. In this way if the terminals of the secondary are widely separated only one of the induced currents has sufficient energy to spark across, so that the secondary discbarge is thus rendered unidirectional. A commutator introduced in the primary circuit enables the operator to make either end of the secondary a plus or a minus pole at will. Induction coils are usually supplied with a spring hammer break tipped with platinum, which automatically makes and breaks the current in the primary, but to-night I shall use instead, a simple foim of mercury bieak. This consists of a tank or small vessel con- taining mercury, into which one end of the primary of the coil dips. A small electric motor is placed in an inclined position, so that its spindle with a small copper blade attached at right angles to it dips down obliquely into the vessel containing the mercury. The battery lias then one of its poles attached to the other end of the primary coil, while the other pole of the battery is attached by means of a spring to the spindle of the motor. It is so placed that as the motor rotates the blade at its end dips into the mercury, and in this way makes good contact and the current of the primary is made; as it ro- tates it comes out of the mercury and so the current in the primary is suddenly broken. To avoid arcing, the mercury is covered to a considerable depth with paraffin or alcohol. In this way we can control the number of breaks which we can produce, and if the motor is running at the same speed, the same number of makes and breaks can be repeated in the same period of time. The speed of the motor is regulated by a small adjustable resistance. The higher the voltage, the greater speed is required in the motor. ] n order to increase the efficiency of the coil a condenser is always introduced as a shunt to the primary across the break terminals. A condenser consists of sheets of tin foil with sheets of paraffin paper between in such a manner that the odd numbers of the sheets of tin foil are all joined together and the even numbers are all joined together, and this arrangement allows the sheets of tin foil to be oppositely charged, and allows the break in the primary with ordinary speeds to be more sudden. Now when I start the break and turn on the current you see the spark leaping across between the terminals. Sparks of this kind were much studied after the invention of the first electric machine about 200 years ago, which produced them, and I will now show you some peculiarities of these sparks. Once a spark has passed through the air and broken it down, it so alters the molecules that it makes it easier for another spark to follow in its wake. Instead of the knobs we take two vertical wires parallel to each other and connect them with the terminals of the coil, and turning on the current you see the spark begins at the bottom of the wires and each succeeding spark takes place higher up than the pre- vious one ; in this way, you have as it were, a ladder of flame. The reason is that the decomposed parts of air by reason of the heat pro-](https://iiif.wellcomecollection.org/image/b21638858_0004.jp2/full/800%2C/0/default.jpg)
