Polytechnic Association. 953 



must be removed sufficient to balance the needle, the amount unplugged 

 must be deducted from the result. 



21. The current is now to be directed through the conductor to be 

 tested, by turning the switch to the right, and the galvanometer 

 deflection noted ; the switch is then turned to the left, directing the 

 current through the rheostat. Plugs are now removed to introduce 

 sufficient resistance to bring the needle to the same degree, so that on 

 oscillating the switch back and forth, the needle remains stationary. 

 The resistance of the conductor is now equal to the sum of the resist- 

 ances of the several rheostat coils introduced. 



By this method any resistance may be directly measured, from ^^ 

 of an ohm to 10,000 ohms. Helices, relays, and other electro-magnets 

 are measured in this way. 



Measuring and Testing Telegraph Lines. 



22. In using this apparatus for testing in a telegraph office, great 

 care should be taken to avoid the disturbing action of masses of iron, 

 the magnets of instruments, and the currents passing through wires, 

 either under the table or along the walls. All the lines should 

 frequently be tested for conductivity and insulation, and the tests 

 recorded in a book kept for that purpose. The records greatly facili- 

 tate the accurate location of faults, whenever it becomes necessary to 

 test for them. 



Testing for Conductivity. 



23. This test should be made in fine weather, when the insulation 

 is good. Have all the relays of the line taken out of circuit, and the 

 line connected to earth or ground wire, at distant end, without bat- 

 tery. Arrange the wires as in last diagram (Fig. 2), putting the line 

 wire and the ground wire to III and IV. When the needle is bal- 

 anced, the resistance unplugged is equal to the resistance of the line. 



No. 9 wire should not exceed twenty ohms per mile of length, and 

 No. 8, seventeen ohms. Higher resistance than this indicates defec- 

 tive joints in the line or poor ground connections. 



In all these measurements and testings, it is proper to reverse the 

 direction of the current through the line (which is readily done by 

 changing places of the connections at I and II), and taking the mean 

 of the two results. 



Testing for Insulation. 



24. The connections are the same as in the last case, except that the 

 line is open at the distant end instead of being " grounded." If the 



