will pull the contact arm back to the start- 

 ing position and the motor will not only- 

 stop but will be protected against the 

 large surge of current which would flow 

 if, when the motor was not revolving, the 

 full voltage were applied across the 

 armature with no starting series resist- 

 ance. 



With the motor brought to full speed 

 by cutting out the starting resistance, 

 the revolutions per minute may be 

 adjusted by use of the motor field 

 rheostat MFR. When this resistance 

 is all cut out, the maximum current flows 

 through the field, and the motor revolves 

 most slowly. Conversely, by cutting in 

 more of the field resistance the motor 

 field current is reduced, the field is weak- 

 ened, and the motor speeds up. If the 

 field is made too weak, the motor will 

 lose power, and though it will run at 

 very high speed when no mechanical load 

 is thrown on it, the speed will be much 

 reduced under load and the operation will 

 be unsatisfactory. Thus it is evident 

 that too much field resistance cannot be 

 used. For a reasonable range, however, 

 the speed may be raised by increasing the 

 resistance in the motor fi_eld circuit. 



The Generator Circuit 



This brings us to the generator G of 

 Fig. 34. The generator is usually 

 mounted upon the same shaft as the 

 motor, as is indicated by the dashed line 

 extending to the right from Fig. 33, and 

 to the left from Fig. 34. Of course the two 

 machines then turn at the same speed, and 

 changing the motor speed by use of the 

 motor field rheostat alters the generator 

 speed correspondingly. 



The generator, like the motor, has two 

 windings; one is for the field and has its 

 terminals marked F Fi, the other is the 

 armature with connections A A\ in 

 Fig. 34. In the generator, however, there 

 is no common terminal; the field wind- 

 ings carry direct current supplied from 

 the DC line through the generator field 

 rheostat GFR, and the armature windings 

 produce the alternating current which is 

 used in the radio transmitter. The fre- 

 quency of this output of alternating cur- 

 rent is determined by the speed of the 

 generator, and may be reduced by slowing 

 down the motor through the motor field 

 rheostat The voltage of the alternating 



current, which may be measured by con- 

 necting an A.C. voltmeter across the 

 armature at A Ai, is varied by changing 

 the strength of the generator field; the 

 stronger the magnetic field, i. e., the less 

 resistance in the rheostat GFR, Hence 

 the greater the field current, the higher 

 the alternating voltage at A Ai. 



Adjustment of Frequency and Voltage 



In addition to the generator itself. 

 Fig. 34 shows the power circuits of the 

 two circuit spark transmitter of Fig. 32, 

 or, in fact, of any sender which uses 

 alternating current to charge a condenser. 

 The armature or output terminals of the 

 alternating current generator G are con- 

 nected through the signaling key K to 

 the primary P of the transformer T. The 

 secondary S of the transformer is con- 

 nected directly across the high-potential 

 condenser C, which in turn may discharge 

 through the wires X F to a spark-gap 

 and inductance coil which are not shown 

 in the figure. By varying the two field 

 rheostats, alternating currents of any 

 frequency and voltage within the range 

 of the apparatus may be applied to the 

 condenser C. The desirability of having 

 such adjustments available will appear 

 when their effects are described in later 

 articles. 



In some coastal commercial radio 

 stations, and in most amateur plants, the 

 alternating current is supplied from a 

 central public service station at a distance. 

 This makes it difficult to adjust the 

 power circuits so as to give the best 

 operation under the most efficient condi- 

 tions, since the frequency cannot be 

 changed and it is hard even to alter the 

 voltage. When power is supplied in this 

 way, and is used without conversion 

 through rotating machinery (which gives 

 a special and controllable generator at 

 the wireless plant), the line conditions 

 usually vary so much from moment to 

 moment that it is not practicable to 

 maintain the rather critical adjustments 

 which give the best results. Consequent- 

 ly, the motor-generator installation is 

 much to be preferred. 



In the next few articles the action of the 

 high voltage and radio frequency circuits, 

 as well as several types of spark gap, will 

 be explained. 



(To he continued.) 



