MAINTENANCE 149 



is found, temporary relief may be provided by inverting the diaphragm when it is 

 replaced. A new one should always be provided as soon as possible. 



(10) The receiver casings are sometimes damaged by expansion and contraction 

 due to changes 1 of temperature. This sometimes causes receiver trouble, which can 

 be remedied only by supplying new casings. 



(11) Inspecting the receiver cord. If the ends are frayed a short circuit often 

 results. If the receiver cord is suspected of being broken it should be removed and 

 tested. It is often very difficult to detect a break in the receiver cord, since it appears 

 intact at one movement, while at the next its continuity is broken. If such difficulty 

 is suspected a new one should be provided. 



(12) Receiver coils are occasionally fused or burned by electrical currents dur- 

 ing lightning storms. When this happens new receivers must be provided. 



(13) The wires of a telephone instrument frequently become damaged, and 

 break. When this 1 occurs it is sometimes possible to detect the trouble only by testing 

 with a buzzer or by the " receiver test." If the wires are intact the buzzer will hum 

 when the wires of the buzzer apparatus are attached to the respective ends of the 

 suspected wire. If the receiver test is applied a click will be heard when the end 

 of the wires leading from the receiver apparatus are applied to the respective ends 

 of the suspected wire. 



(14) Induction coils. Fortunately, faults in induction coils seldom occur. They 

 are, however, not immune from damage and sometimes are burned by electrical cur- 

 rents during lightning storms, or if the line wire leading to the instrument comes 

 in contact with transmission lines carrying high-voltage currents. When damaged, 

 a new one must be supplied. 



A word of caution, however, must be spoken in regard to the tampering with 

 telephone apparatus of those who are entirely inexperienced and ignorant of the 

 mechanism of the equipment. Hasty action may cause much more serious difficulty 

 than that which it is undertaken to correct. Repairs should be undertaken cautiously. 

 If the apparatus does not work well do not meddle with it until you have located the 

 seat of the trouble and know just what to do to correct it. If the fault cannot be 

 found, the instrument should be removed and placed in the hands of an expert for 

 repairs. 



In most cases it is advisable to send the instrument to the nearest office of the 

 manufacturer. To enable this to be done without interruption of the service, one or 

 two extra instruments should be kept on every forest reserve and installed temporarily 

 in the place of any that are undergoing repairs. 



2 BATTERIES 



All dry batteries on the line should be replaced at least every twelve months, and 

 more often if necessary. As far as practicable, all the batteries on a line should be 

 renewed at the same time, preferably at the beginning of the fire season. A fresh 

 battery should not be connected to an old one. The inspector should note whether or 

 not battery connections have become loosened. In putting dry batteries back into a 

 telephone, the inspector should see that the zinc binding-post on one cell does not 

 touch the zinc binding-post on an adjacent cell (Fig. 43). 



An emergency test to determine whether or not a dry cell is absolutely dfead may 

 be made by moistening the finger and gripping the zinc binding-pest, and then touch- 

 ing the tip of the tongue to the carbon binding-post. If a very slight acidulous taste 

 is noted the cell is not absolutely exhausted, although it may be sufficiently so to be 

 incapable of giving good transmission. In extreme emergencies exhausted dry cells 

 can sometimes be temporarily revived to give, sufficient current for sending an 

 important message over the line 'by driving holes with a nail through the zinc shell 

 and allowing water to soak in through them. 



