194 POPULAR SCIENCE MONTHLY. 



Very great care is required in the insulation of the secondary 

 circuit of an induction coil to be used in Hertzian wave telegraphy, be- 

 cause the secondary circuit is then subjected to impulsive electro- 

 motive forces lasting for a short time, having a much higher electro- 

 motive force than that which the coil itself normally produces. 



The primary circuit of a ten-inch coil generally consists of a length 

 of 300 or 400 feet of thick insulated copper wire. In such a coil the 

 secondary circuit would require about ten miles of No. 34 H.C. copper 

 wire, making 50,000 turns round the core. It would have a resistance 

 at ordinary temperatures of 6,600 ohms, and an inductance of 460 

 henrys. The primary circuit, if formed of 360 turns of No. 12 H.C. 

 copper wire, would have a resistance of 0.36 of an ohm, and an induct- 

 ance of 0.02 of a henry. 



An important matter in connection with an induction coil to be 

 used for wireless telegraphy is the resistance of the secondary circuit. 

 The purpose for which we employ the coil is to charge a condenser of 

 some kind. If a constant electromotive force (V) is applied to the 

 terminals of a condenser having a capacity C, then the difference of 

 potential (v) of the terminals of the condenser at any time that the 

 contact is made is given by the expression : 



i; = F(l— 6~"^) 



In the above equation, the letter e stands for the number 2.71828, 

 the base of the Napierian logarithms, and R is the resistance in series 

 with the condenser, of which the capacity is C, to which the electro- 

 motive force is applied. This equation can easily be deduced from 

 first principles,* and it shows that the potential difference v of the 

 terminals of the condenser does not instantly attain a value equal to 

 the impressed electromotive force Vj but rises up gradually. Thus, for 

 instance, suppose that a condenser of one microfarad is being charged 

 through a resistance of one megohm by an impressed voltage of 100 

 volts, the equation shows that at the end of the first second after con- 

 tact, the terminal potential difference of the condenser will be only 

 63 volts, at the end of the second second, 86 volts, and so on. 



Since c~iois an exceedingly small number, it follows that in ten 

 seconds the condenser would be practically charged with a voltage equal 

 to 100 volts. The product CR in the above equation is called the 



Also see Vol. II. of ' The Alternate Current Transformer/ by J. A. 

 Fleming, Chap. I. (The Electrician Publishing Co., 1, Salisbury Court, Fleet 

 St., London, E. C.) 



* See ' The Alternate Current Transformer,' by J. A. Fleming, Vol. I., 

 page 184. 



