quency desired is delivered from the 

 armature terminals of the machine. 



Let us consider a typical spark-sender 

 installation such as is used aboard ship 

 and at many land stations. Direct cur- 

 rent electric power is provided from the 

 engine room (or by a public service 

 corporation), and wired to the radio 

 station. Here it passes to a control 

 switchboard and a motor-starter, which 

 is associated with the direct-current 

 motor used to drive the alternating cur- 

 rent generator for the radio transmitter. 

 In Fig. 33 the power circuits of such an 

 installation are shown, reduced to their 

 simplest form. The direct current line 

 comes in at Li and L-z, and usually 

 operates at a voltage of 110 or 120. As 

 shown in the diagram, the lines lead 

 directly to two fuse-wires Wi Wo, which 

 serve to protect the apparatus by "blow- 

 ing" or melting in case too much current 

 is drawn from the line (by reason of a 

 short-circuit or other abnormal condi- 

 tion). From the fuses, the connection 

 runs to a double-pole line switch LS, 

 which is used to disconnect the entire 

 motor circuit when the plant is not 

 running. The motor is usually stopped 

 by pulling this switch open. When the 

 smtch is closed, the power is applied 

 across the terminals of a voltmeter VM, 

 which will always show the voltage of the 

 direct current line if the fuses are in good 

 condition and if the circuits are all right 

 up to this point. From the voltmeter the 



Fig. 33 : In most modern installations an au- 

 tomatic electrically-controlled starter is used 



wires run to the motor itself, shown at the 

 right of the diagram Fig. 33, passing 

 through a motor field rheostat MFR and 

 a starting box SB. The starting box 

 illustrated is of the simple hand-con- 

 trolled type. In most modern installa- 

 tions an automatic, electrically-controlled 



starter is used, but the principle is the 

 same. 



Connections of the Motor 



It will be noted that the motor is 

 shown with two field terminals marked 

 F and Fx and two armature connections 

 A and A^. The armature connections 



Fig. 34: In addition to the generator itself 

 the circuits of the transmitter are shown 



lead to the brushes, as shown, and current 

 applied to them reaches the low re- 

 sistance armature windings through the 

 rotating commutator. The field is a high 

 resistance stationary winding connected 

 directly across the direct current line 

 through the terminals F Fi. It is often 

 called "shunt field" to distinguish it 

 from a low resistance winding called the 

 "series field" which is sometimes used, on 

 compound-wound motors, in series with 

 the armature. One terminal of the field 

 and one of the armature are usually 

 brought together at a "common" point, as 

 illustrated by A F in the figure. 



The starting box SB contains a 

 variable resistance which is connected in 

 series with the low-resistance armature 

 windings. As the contact arm is pulled 

 over slowly, less and less of the resistance 

 remains in the circuit, and the motor 

 turns over faster and faster until (when 

 the line is directly across the armature 

 and all the starting resistance is cut out) 

 full speed is reached. On most starting 

 boxes the contact arm is provided with 

 a spring tending to hold it in the initial or 

 high-resistance position, and a small 

 retaining-magnet which keeps it in the 

 running position so long as the line 

 voltage is normally high. If the current 

 is cut off by opening the line switch, LS, 

 or through an interruption of the power 

 circuit outside the radio station, this 

 retaining magnet (often called a "no- 

 voltage release") will let go, the spring 



