160 



Popular Science Monthly 



Magnetic Brake for a Wireless 

 Rotary Gap 



THE radio experimenter who uses a 

 rotary spark gap in connection with 

 his sending apparatus is usually troubled 

 with interference in his receiving set caused 



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Eliminating the interference of inductive 

 noises from the motor of a rotary gap 



by the inductive noises from the motor of 

 the rotary gap, which if well balanced takes 

 some time to come to a full stop. 



In the drawing, A represents the blade 

 of a single pole, double throw, switch. The 

 figures B and C are the two jaws of the 

 switch. The figure D represents a rheostat 

 by means of which the length of time nec- 

 essary for the motor to come to a full stop 

 may be regulated. At E and F are the 

 fields and armature of a series-wound 

 motor. 



The action is as follows: To start the 

 motor, throw the blade A to contact C. 

 When through sending, throw blade to con- 

 tact B, which causes the current from the 

 line to flow to one field through the rheostat 

 D, and results in stopping the motor in two 

 to three seconds. After the motor has 

 stopped, disengage the switch blade from 

 jaw B, otherwise a waste of current will 

 result. — Paul J. Hoffman. 



Adjusting the Detector of a 

 Receiving Set 



WHEN the crystal or other detector of 

 a wireless telegraph receiver is ad- 

 justed by the use of an ordinary buzzer set 

 up near the instruments, it is often noted 

 that the point of contact which gave loudest 

 response to the buzzer is not that which is 

 most sensitive for receiving signals from 

 long distances. The most sensitive spots 

 sometimes do not give loud sounds when 

 the local buzzer is operated. 



This has been noted by many experi- 

 menters who have electrolytic and crystal 

 detectors in use side by side; generally the 

 crystal will give the loudest signal when the 



buzzer is worked, but the electrolytic will 

 prove better for receiving from stations far 

 away. This is because the character of the 

 test impulses produced by the ordinary 

 buzzer is quite different from the radio fre- 

 quency-currents set up in the receiving 

 aerial by the distant station. 



In a patent (No. 1,176,925) issued during 

 1916 to G. W. Pickard, there is shown a 

 method of avoiding this difficulty. As in- 

 dicated in the drawing here reproduced, a 

 buzzer having armature A, contact B and 

 magnet C is connected in series with bat- 

 tery D and test key E. Across the vibrat- 

 ing contact is shunted a high-frequency 

 oscillating circuit comprising the condenser 

 F and the inductance G. This last named 

 element is coupled variably to the sec- 

 ondary H of the receiving oscillation- 

 transformer, which has the usual tuning 

 condenser, detector, blocking condenser, 

 telephones and potentiometer arranged as 

 shown. The shunt oscillation circuit F, 

 G, is adjusted to produce feebly damped 

 groups of radio frequency-current corre- 

 sponding to the wavelength most used at 

 the receiver. 



When the buzzer is put into operation 

 by pressing the key E, there are generated 

 in the transformer secondary radio fre- 

 quency-currents corresponding to those re- 

 ceived in actual radio telegraphic practice. 

 The groups produce tone signals, of the 

 buzzer interruption-frequency, in the tele- 

 phones. The loudness of these signals de- 

 pends upon the coupling between the coil 



Diagram of connections for buzzer exciter 

 which permits accurate setting of the crystal 



G and the secondary, and upon the true 

 sensitiveness of the detector. By selecting 

 the point of crystal which gives loudest 

 responses to such excitation, when the buz- 

 zer coupling is set to produce an intensity 

 corresponding to that of the station which 

 it is desired to receive, the operator may 

 have entire confidence that his detector is 

 properly prepared to do the best work. 



