134 



METHODS OF COMMUNICATION FOR FOREST PROTECTION 



through the long coils of the telephone ringer or other parts of the set. This arrester 

 (Fig. 83) consists of a suitable insulating block A on which 'are placed two pairs of 

 copper blocks B, B 1 held together by springs but not in direct contact with each other 

 because of a thin sheet of mica placed between them. A brass bar extends from 

 binding-posts and C 1 to the copper blocks nearest them. A similar bar extends 

 from the inner blocks to binding-post D. The line wire is connected to post C and the 

 ground wire to post D. To all ordinary telephone currents the air gap between the 

 copper blocks is an effectual bar. Dust collecting between the blocks caus2S a short 

 circuit but is largely prevented from accumulating by the brass covering cap E. By 



Fig. 84 Protector with fuses, for use where there 

 are power and light wires. Note 

 method ' of binding edge of asbestos 

 mat with metal tape 



a lightning current, however, the path to the ground across the air gap is taken in 

 preference to that through the instrument, and in this way such currents are success- 

 fully prevented from damaging the telephones. 



The effect of lightning-arresters on high-tension currents other than lightning 

 is somewhat similar, but these currents are very likely to be continuous instead of 

 merely momentary as are lightning currents. A " cross " with such a current would 

 therefore result in a constant sparking across the air gap of the protector, which 

 would almost certainly cause a fire if it did no other serious damage. To prevent 



