. The practical telephone handbook and guide to the telephonic exchange . g to metallic cir-cuit lines having no directconnection with earth,they are especially liableto damage by lightning,so that with such lines,in addition to the arres-ters forming part of theinstruments, it is usualto provide other safetydevices, both at the ex-change end of the lineand at the point at whichthe line wires enter the sub-station. These are made in theform of window terminals, serving also to terminate theouter line wires. Fig 105 shows a common form, in whichthe earth plate of the arrester is of the saw-toot
. The practical telephone handbook and guide to the telephonic exchange . g to metallic cir-cuit lines having no directconnection with earth,they are especially liableto damage by lightning,so that with such lines,in addition to the arres-ters forming part of theinstruments, it is usualto provide other safetydevices, both at the ex-change end of the lineand at the point at whichthe line wires enter the sub-station. These are made in theform of window terminals, serving also to terminate theouter line wires. Fig 105 shows a common form, in whichthe earth plate of the arrester is of the saw-tooth form, ad-justed to the two brass-line terminal plates. It is necessaryto provide a good earth connection. Special Protection.—Where the lines are in the least likely tocome into the neighbourhood of heavy-current supply lines.,which are now very common, safety devices of a more elaboratenature are necessary to obviate the possibility of troubleresulting from an accidental contact with the current supplywires. Such contact may be (a) of somewhat high resistance, but. Fig. 105 106 PRACTICAL TELEPHONE HANDBOOK affording passage to a current of a strength which, if con-tinued for a few minutes, will raise the temperature of thincopper wire—such as that of bell coils, etc.—until it is hotenough to ignite surrounding materials. These are calledsneak currents in America, and are guarded against by pro-viding special thin wire coilsof German silver wire, andutilising the heat developedby the current to fuse a por-tion of easily fusible metal,this releasing a spring whichbreaks the circuit. Or (b) the contact may beof low resistance, and the cur-rent may more likely be of astrong and dangerous such risks a fusible metalwire, which immediately fuseswhen the current is from 2Jto 3 amperes, is inserted inthe circuit. Another case is (c) wherethe potential of the line maybe raised to sparking point;but very little current is pro-duced, both wires being at anearly equal po
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Keywords: ., bookcentury1900, bookdecade1900, booksubjecttelephone, bookyear19
