HERTZIAN ^yAVE WIRELESS TELEGRAPHY. 195 



iime-constant of the condenser, and we may say that the condenser is 

 practically charged after an interval of time equal to ten times the 

 time-constant, counting from the moment of first contact between the 

 condenser and the source of constant voltage. The time-constant is to 

 be reckoned as the product of the capacity (C) in microfarads, by the 

 resistance of the charging circuit {R) in megohms. To take another 

 illustration. Supposing we are charging a condenser having a capacity 

 of one hundredth of a microfarad, through a resistance of ten thousand 

 ohms. Since ten thousand ohms is equal to one hundredth of a 

 megohm, the time-constant would be equal to one ten-thousandth of a 

 second, and ten times this time-constant would be equal to a thou- 

 sandth of a second. Hence in order to charge the above capacity 

 through the above resistance, it is necessary that the contact between 

 the source of voltage and the condenser should be maintained for at 

 least one thousandth part of a second. 



In discussing the methods of interrupting the circuit, we shall re- 

 tarn to this matter, but, meanwhile, it may be said that in order to 

 secure a small time-constant for the charging circuit, it is desirable that 

 the secondary circuit of the induction coil should have as low a re- 

 sistance as possible. This, of course, involves winding the secondary^ 

 circuit with a rather thick wire. If, however, we employ a wire larger 

 in size than No. 34, or at the most No. 32, the bulk and the cost of the 

 induction coil began to rise very rapidly. Hence, as in all other de- 

 partments of electrical construction, the details of the design are more 

 or less a matter of compromise. Generally speaking, however, it may 

 be said that the larger the capacity which is to be charged, the lower 

 should be the resistance of the secondary circuit of the induction coil. 



In the practical construction of induction coils for wireless teleg- 

 raphy, manufacturers have departed from the stock designs. We are 

 all familiar vdth the appearance of the instrument maker's induction 

 coil; its polished mahogany base, its lacquered brass fittings, and its 

 secondary bobbin constructed of and. covered with ebonite. But such 

 a coil, although it may look very pretty on the lecture table, is yet very 

 unsuited to positions in which it may be used in connection with 

 Hertzian wave telegraphy. 



Three important adjuncts of the induction coil are the primary 

 condenser, the interrupter and the primary key. The interrupter is 

 the arrangement for intermitting the primary current. We have in 

 some way or other to rapidly interrupt the primary current, and the 

 torrent of sparks that then appears between the secondary terminals 

 of the coil is due to the electromotive force set up in the secondary cir- 

 cuit at each break or interruption of the primary circuit. We may 

 divide interrupters into five classes. 



