Page:Popular Science Monthly Volume 91.djvu/176

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��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.

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