. Development and electrical distribution of water power . Fig. 74. to the circuit, and the voltmeter is connected to the secondary sideof the transformer. Fig. 74 shows a three-phase line with theprimaries of three small transformers connected across the three 148 DEVELOPMENT AND DISTRIBUTION OF WATER POWER phases. The secondaries leave the three small switches whichlatter connect to a pair of wires leading to the voltmeter. Bythrowing in any switch a single voltmeter is made to indicate thevoltage of each of the three phases. Where three-phase systemsare balanced—that is, equal current passi
. Development and electrical distribution of water power . Fig. 74. to the circuit, and the voltmeter is connected to the secondary sideof the transformer. Fig. 74 shows a three-phase line with theprimaries of three small transformers connected across the three 148 DEVELOPMENT AND DISTRIBUTION OF WATER POWER phases. The secondaries leave the three small switches whichlatter connect to a pair of wires leading to the voltmeter. Bythrowing in any switch a single voltmeter is made to indicate thevoltage of each of the three phases. Where three-phase systemsare balanced—that is, equal current passing over each of the threewires—it is necessary only to take the voltage of one of the phases,as the voltages of all the phases will all be equal. This is thecondition existing in nearly all transmission systems; and inthem only one transformer with its secondary connected to thevoltmeter is required. Ammeters on high-tension circuits are also connected to trans-. Fig. 75. formers in which the primary consists of one or two turns in serieswith the main current. The secondary consists of a number ofturns, its terminals being connected to the ammeter. Obviously,the volts generated in the secondary will be proportional to thecurrent in the main line passing through the primary. The in-strument itself is in reality a voltmeter; but the movements of itsneedle being proportional to the current passing in the main line,the markings of its dial are in amperes. Fig. 75 shows the con-nections. Wattmeters record the quantity of electrical energy generated; SWITCHING AND CONTROLLING APPARATUS 149 and the usual type of integrating instrument makes a continuousrecord of total energy delivered over a certain period of have two windings, one a shunt, the other a series winding;and therefore, potential and series transformers both are requiredfor them. Circuit breakers which work on high-tension lines must takecurrent for the tripping coils from seri
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