. Electrical instruments and telephones of the Signal corps . iriO ohms, the sameas other main-line instruments. The pocket relay is a compact form of main-line sounder fortesting purposes. About 40 millianiDeres current is required to operate the 150-ohminstrument to bestadvantage. THE SOUXDEB. This Avell - knowninstrument is shownin figure 29, this be-ing one of the mostcouunon forms nowin use. Its connec-tion with the relayand local battery cir-cuit has alreadj^ beenindicated. The coils are usually wound to a resistance of 4 ohms, andit requires about one-fourth ampere to operate the s
. Electrical instruments and telephones of the Signal corps . iriO ohms, the sameas other main-line instruments. The pocket relay is a compact form of main-line sounder fortesting purposes. About 40 millianiDeres current is required to operate the 150-ohminstrument to bestadvantage. THE SOUXDEB. This Avell - knowninstrument is shownin figure 29, this be-ing one of the mostcouunon forms nowin use. Its connec-tion with the relayand local battery cir-cuit has alreadj^ beenindicated. The coils are usually wound to a resistance of 4 ohms, andit requires about one-fourth ampere to operate the sounder as vigor-ously as is required for ordinary offices. Hence two bluestone cells,each having about 1 volt E ]M F and 2 ohms internal resistance, willgive the required current. E 2 (6=-; .25 = 2^2 + 4-^ If the sounder is put in circuit with a cell of higher E M F andlower resistance, some resistance wire should l)e put in to keep thecurrent down to one-fourth ampere: otherwise the battery will beused up wastefully. For example, with a Fuller cell of volts. ■Fk;. E M F and one-fourth ohm internal resistance, 6^= j-^ = .42 ampere, 4+4 which is considerably more current than necessary. The necessaryadded resistance, A, can be found as follows: Required current is .25 1 S ampere, so .25=:;—^ t>; + .25 A=; A^=3 ohms, the dead ^ 4+i+A resistance to be introduced in circuit. In cases when a number of sounders are fed from one storage sell. a dead resistance* of 4 ohms should be inserted in each sounder circuit, as the storage cell has virtually no resistance and an E M F of 2 volts. ELECTRICAL INSTRUMENTS U. S. SIGNAL CORPS. 65 SWITCHBOARDS. These are either terminal or intermediate in their use, and as thereis considerable difference in extent and character of the wiring, theywill be considered separately. The intermediate or way-station switchboard is represented insimplest form in figure 30. Suppose two lines coming into a stationare brought to the to
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