132 METHODS OF COMMUNICATION FOR FOREST PROTECTION 



when several persons are " listening in," were it not for the use of a condenser in the 

 receiver circuit. All standard Forestry Branch instruments are thus equipped, the 

 wiring being as shown in Figs. 41 and 97. 



The condensers used for this purpose consist of alternate strips of tin-foil separated 

 from each other by waxed paper. They have a capacity of half a microfarad. A 

 condenser acts as a complete bar to the passage of direct currents, or, in other words, it 

 constitutes an open circuit. To alternating currents, however, it offers a more or less 

 perfect path depending on the character of the current, particularly its frequency and 

 the capacity of the condenser. The condensers used in the standard instruments readily 

 permit the passage of the high-frequency currents of the talking circuit but bar out the 

 low-frequency currents of the calling circuit almost completely. It is thus made 

 possible to ring through, even though a considerable number of receivers may be off the 

 hooks. 



Fig. 82 Howler 



Similarly, a condenser is used with the howler where vibratory currents are 

 employed in signalling as with the 1004-A hand set. These vibratory currents are 

 alternating currents of very high frequency which readily pass through the 1-micro- 

 farad condenser, wired in series with the howler. This condenser, however, effectively 

 prevents the escape to the ground of the low-frequency alternating currents of the 

 magneto generators, so that both types of signalling devices may be used on the one 

 line without short circuiting or interference. 



(fr) Howler. For the purpose of reducing the weight of the portable instruments 

 as much as possible it was necessary to do away with the heavy magneto generator. 



