TELEPHONES 459 



blocks being separated by a thin piece of mica, a part of 

 which is cut away, thus leaving an air gap. One of the 

 blocks is in electrical contact with a line wire and the other 

 is connected with the earth. Currents which may be dan- 

 gerous, either to the telephone instruments or to their users, 

 will tend to discharge across the space between the carbon 

 blocks. 



A satisfactory ground connection for the lightning arrestor 

 may be made by means of a 6-foot iron rod, having a wire 

 soldered to it, driven into moist earth as a lightning rod is 

 grounded. The connection from the lightning arrestor to 

 the ground rod should be made by means of No. 14 B & S 

 gauge braided rubber-covered copper wire, run as straight 

 as possible from rod to arrestor, and protected against 

 woodwork by porcelain knobs and tubes, as this wire is 

 also subject to heating. 



From the lightning arrestor to the telephone instrument, 

 No. 18 gauge twisted-pair braided, rubber-covered house 

 wire should be used. 



The type of telephone instrument to be selected will 

 depend largely on the kind of service which is decided upon. 

 It would be well to consult with the representatives of 

 reliable manufacturers of telephones, and, with the useful 

 data which they can contribute, to make a selection of 

 instruments which will fulfill the requirements. 



SUMMARY 



Briefly summarized, the important steps to be taken 

 preparatory to building a privately owned telephone line 

 are as follows: 



1. Determine whether the line is to be isolated, or a part 

 of an exchange system. 



(a) If the former, is it to be metallic or a ground circuit? 



(6) If the latter, is it to be a metallic or ground circuit, 

 local or common battery, selective, semi-selective, or non- 

 selective (code-ringing) type? 



