144 



Electric Transformers. 



No. 50668 



Direct-Direct Current Transformers. 



These transformers are single-arma- 

 ture transformers and are employed in 

 cases where a 220, 440 or 500 volt Direct 

 Current network is available and where it 

 is not wished to let the superfluous current 

 in connection with the working of the pro- 

 jection lantern or for experimental pur- 

 poses go to waste through the series re- 

 sistances which must be used for reducing 

 the current. With high tensions, particu- 

 larly, the nullification of the current through 

 series resistances gives rise to heavy working 

 costs. In addition, the leads must in this 

 case be dimensioned for the full current, 

 while if a transformer is installed they can 

 be dimensioned for a proportionately low 

 current. The transformers yield a current 

 of 65 volts Direct Current sufficient for the 

 projection lantern and for experimental pur- 

 poses. 



An experimental switchboard, preferably of the A, -type, is connected up with' the secondary 

 terminals of the dynamo. It is possible with the aid of this board to take off any desired current-strength 

 as well as low potentials. 



It is best to erect the transformer in the preparation room or any other room adjoining the 

 class room so as to avoid any disturbance due to the noise of its running. 



As suggested by Prof. Friedr. C. G. Miiller (M. T., p. 248) the experimental plant can be arranged 

 by connecting the experimental switchboard and projection lantern direct to the 220 volts network and 

 providing a small transformer of about 100 watts output (5 amps, at 20 volts see Nos. 50,677 50,680) 

 for experiments with low tension currents, especially for charging a 6- or 8-cell battery. 



50 670. 



Direct-Direct Current Transformers, Figure, 

 to about 65 volts. 



for reducing the voltage of direct current at 220 volts 



*) When ordering, in addition to quoting the list number, kindly also mention the letters a, b, c, etc. placed 

 in front of the price lines. 



These single-armature transformers have a commutator on each of the two ends: one being used for taking 

 the current from the network and the other for taking the transformed current from the machine for experimental 

 purposes. 



The operation of starting is as simple as in the ordinary direct current motor; thus it is only necessary to move 

 the lever of the starting rheostat and the pressure of 65 volts or less is immediately available. A starting rheostat 

 for starting the machine must be ordered with the transformer. The starting rheostats included in the table permit 

 of the machine starting on half load. For regulating the voltage a Starting and Regulating Rheostat can be em- 

 ployed in lieu of the simple starting rheostat; in this case the speed at full load can be reduced to 50% 

 of that given. 



The Direct-Direct Current Transformers are connected up to the network in the same manner as ordinary 

 shunt motors. The transformers require no transmission drive and are therefore as a rule supplied without belt pulley ; 

 they can also be used as 220 volt Direct Current Motors. 



C J Jl 



