1902.] on X-Bays and Localisation. 125 



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 

 discharge is thus rendered unidirectional. A commutator introduced 

 in the i)rimary 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 

 form of mercury break. 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 

 has 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 dijDS 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 sjDeed, 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. 



In 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 sj)ark8. 



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 cuirent 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- 



