[Electric engineering.] . dingly fineadjustment of E. M. F. It may be mentioned, in passing,that the field rheostat used in connection with a compound-wound machine is not usually varied to any great extentafter the machine has settled down to its normal the dynamo is first started up, its fields are cold, butafter it has been running a while, they warm up and theirresistance increases so that it is necessary to cut out someof the rheostat in order to keep up the voltage. However,after the fields have reached their normal temperature, thecompound coils will generally take care o
[Electric engineering.] . dingly fineadjustment of E. M. F. It may be mentioned, in passing,that the field rheostat used in connection with a compound-wound machine is not usually varied to any great extentafter the machine has settled down to its normal the dynamo is first started up, its fields are cold, butafter it has been running a while, they warm up and theirresistance increases so that it is necessary to cut out someof the rheostat in order to keep up the voltage. However,after the fields have reached their normal temperature, thecompound coils will generally take care of the voltage, andvery little further adjustment of the field rheostat is neces-sary. Field rheostats are made in many different styles,and a great many different schemes have been adopted formaking up and mounting the resistance. Fig. 33 shows the appearance of a typical field rheostatmounted in an iron box and arranged for attachment to thefront of the switchboard. The resistance is here divided 32 ELECTRIC LIGHTING. 17. Fig. 33. into 52 sections connected to the contact points / mountedon the slate front. By turning the hand wheel H, any num-ber of sections maybe cut in or all rheostatsare made so thatturning the handleto the right will cutin resistance andlower the voltage ofthe dynamo. Thisis the same direc-tion that an ordi-nary globe valve isturned to shut offthe steam or most modernslate or marbleboards, the rheostat is mounted on the back of the panel,and all that appears on the frontis the hand-wheel necessary foroperating it. Fig. 34 shows a rheostat ar-ranged in this way. The oneshown is of the enamel type,in which the wire is made upinto flat zigzag coils and embed-ded in enamel on the back ofa ribbed iron plate. This holdsthe wire securely in place andat the same time allows it toimpart the heat generated to theribbed iron plate, which radiatesit. The figure shows the oper-ating wheel H that moves the fig. a over the contacts b ; t, f ar
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