2o6 POPULAR SCIENCE MONTHLY. 



former as a source of electromotive force, although it may have the 

 advantage that the resistance of the secondary circuit of the trans- 

 former is generally small compared with that of the secondary circuit 

 of an induction coil, yet nevertheless we are confronted with two 

 practical difficulties: (1) How to control the primary current flowing 

 into the transformer; and (2) how to destroy the alternating current 

 arc between the spark balls and reduce the discharge entirely to the 

 disruptive or oscillatory discharge of the condenser. 



The control over the current can be obtained, in accordance with 

 a plan suggested by the author, by inserting in the primary circuit of 

 the transformer two variable choking coils. The form in which it is 

 preferred to construct these is that of a cylindrical bobbin standing 

 upon a laminated cross-piece of iron. These bobbins can have let 

 down into them an E-shaped piece of laminated iron, so as to com- 

 plete the magnetic circuit, and thus raise the inductance of the bob- 

 bin. By placing two of these variable choking coils in series with 

 the primary circuit, the current is under perfect control. We can 

 fix a minimum value below which the current shall not fall, by adjust- 

 ing the position of the cores of these two choking coils, and we can 

 then cause that current to be increased up to a certain limit which 

 it can not exceed, by short-circuiting one of these choking coils by 

 an appropriate switch. Several ways have been suggested for extin- 

 guishing the alternating current arc which forms between the spark 

 balls connected to the secondary terminals when these are brought 

 within a certain distance of one another. One of these is due to Mr. 

 Tesla. He places a strong electromagnet so that its lines of magnetic 

 flux pass transversely between the spark balls. When the discharge 

 takes place the electric arc is blown out, but if the balls are short- 

 circuited by a condenser, the oscillatory discharge of the condenser 

 still takes place across the spark gap. Professor Elihu Thomson 

 achieves the same result by employing a blast of air thrown on the 

 spark gap. This has the effect of destroying the alternating current 

 arc, but still leaves the oscillating discharge of the condenser. The 

 action is somewhat tedious to explain in words, but it can easily be 

 understood that the blast of air, by continually breaking down the 

 alternating current arc which tends to form, allows the condenser con- 

 nected to the spark balls to become charged with the potential of the 

 secondary circuit of the transformer, and that this condenser then 

 discharges across the spark gap, producing an oscillatory discharge in 

 the usual manner. The author has found that without the use of 

 any air blast or electromagnet, simple adjustment of the double cho- 

 king coil in the primary circuit of the transformer as above described 

 is sufficient to bring about the desired result, when the capacity of the 

 condenser is adjusted to be in resonance. 



