INSTALLATION OF TEMPORARY EQUIPMENT 



107 



circuits, the number and position of all telegraph stations on the line it is proposed 

 to- use, the size and kind of wire and whether the telegraph operates by single or double 

 current, simplex, duplex, quadruplex, or automatic. 



Telegraph Instrument 



_cT^ 



Battery -=- 



Jeleyraph Instrument 



f ^U- 



00 



00 



Fig. 61 Method of using one wire for simultaneous telegraphy and telephony 



Bell signals cannot be employed on superimposed circuits so that most of the 

 standard Forestry Branch telephone instruments cannot be used on such lines without 

 modification. The 1004- A set which employs buzzer signals may be used, however. 



In the majority of cases the object will probably be to superimpose a grounded 

 telephone circuit on a grounded single-current telegraph circuit. The telegraph 

 employs direct current while the telephone employs alternating current. If, therefore, 

 condensers are inserted between the telephone rnstruments and the line, there will be no 

 tendency for the telegraph current to escape to ground through them, while on the 

 other hand the alternating currents of the telephone are not of sufficient strength to 

 affect the telegraph instruments. The simplest form of such a superimposition is 

 shown diagrammatically in Fig. 61. 



CHAPTER XIII 



INSTALLATION OF TEMPORARY EQUIPMENT 



Section 79 Types of Portable Sets 



Portable telephones are used for two principal purposes. To a certain extent 

 they are required for testing lines, both during construction and in maintenance opera- 

 tions. Portable sets used for these purposes are usually known as " test sets." These 

 are on the market in great variety, each type being adapted by the details of its con- 

 struction to some particular class of work. Some of these sets are very low-priced and 

 light in weight, but none of them meets successfully the special problems ordinarily 

 encountered on forest protection lines. 



In. addition to their use for testing lines, portables are required in forest protection 

 for establishing temporary emergency stations on or adjacent to the permanent lines of 

 communication. The average conditions met with on forest telephone lines demand a 

 portable set of considerable power, especially in the generator. This instrument must 

 approximate the capabilities of the very powerful standard sets employed in the per- 

 manent stations and must be adapted for use on lines with them. Furthermore, since 



