. Electrical instruments and telephones of the Signal corps . currents, asshown in the foreffoinir tests. ELECTRICAL INSTRUMENTS U. S. SIGNAL CORPS. 249 THE VOLTMETER AND AMMETER. On land telegraph lines and the apparatus connected therewith theelectrical units Avith which we are usually concerned iu measurementsand tests are those given in Ohms law—the current in amperes equalsthe electromotiA^e force in volts divided by the resistance of the cir-cuit in ohms; expressed algebraically C=-^. The galvanometer, in one or the other of its forms, uieasures current. AVhen of low resist-ance and
. Electrical instruments and telephones of the Signal corps . currents, asshown in the foreffoinir tests. ELECTRICAL INSTRUMENTS U. S. SIGNAL CORPS. 249 THE VOLTMETER AND AMMETER. On land telegraph lines and the apparatus connected therewith theelectrical units Avith which we are usually concerned iu measurementsand tests are those given in Ohms law—the current in amperes equalsthe electromotiA^e force in volts divided by the resistance of the cir-cuit in ohms; expressed algebraically C=-^. The galvanometer, in one or the other of its forms, uieasures current. AVhen of low resist-ance and graduated properly, it is called an amperemeter or of high resistance, since the current flowing through it is prac-tically independent of the relatively small variations of outside re-sistance, the galvanometer readings are directly proportional to theelectromotive force E. And when properly graduated it becomes avoltmeter. The ammeter and voltmeter, on account of portability andquickness and accuracy wath which readings are taken, are very sat-. FiG. 165. isfactory instruments for telegraph testing. It is evident if C andE are measured by an ammeter and voltmeter, respectively, that R becomes known—for Ohms law may be written J?=-=^. For example, if we connect the ammeter .1, battery B^ and a resistance coil Ctogether, as in figure 1G5, we may read the current flowing. Thesmall current commonly used in telegraphy is conveniently expressedin milliamperes, and the ammeter graduated for these is called themilli- or mil-annneter. If we attach a voltmeter V to the terminals ofthe resista)ice coil C, it will giAe the difference of potential (E M F)produced at these two points by the current flowing between the milliammeter reads 28 milliamperes ( ampere) E and the voltmeter volts. Substituting ini?=-^„ Il=-^r^ = \h\ ohms. The general rule in connecting up the ammeter and voltmeterfor such measurements is to put the annneter in the cir
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