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52
RADIO BROADCAST
ness will operate successfully, and exceptional results have been obtained with Western Electric Vt's one and two.
Modulation of extraordinary quality is secured by shunting a partial turn of the loop by a microphone, one terminal of which is connected on the grid side of the loop while the other is clipped over that fraction of a turn (usually about one half) which gives best speech, i.e., the loudest voice without distortion or blocking of the set.
Constructional details will be immediately comprehended by reference to figure two. The
SIDE VIEW
Fig. 2
instruments (socket, condensers, fixed and variable, and rheostat) are symmetrically arranged on a ten-inch square base about the upright pole which supports the loop. The weight and size of the complete equipment are such that the whole is easily revolved to secure directional transmission.
The experimenter will find many and varied uses for this apparatus in laboratory or research work, and 1 have used the set quite extensively as a calibrating wavemeter. To do this it is necessary to determine first the frequencies at which the transmitter oscillates at different capacities as indicated by the condenser scale. Employed in this manner, the set is extremely useful in calibrating other wavemeters and receiving sets. In the latter case it is advisable to dispense with the micro
phone and tune to a zero beat with the receiving set oscillating. The microaccuracy of this method is immediately apparent.
For intercommunicative work covering a distance which will vary directly with the applied plate potential from one to several blocks, the connections should be slightly altered so that they appear as in figure three. It will be observed in this circuit that a telephone headset and a high resistance potentiometer R, with the shunt push-button P, are placed in series with the high voltage. With the push-button released, the combined reactance and resistance of the phones and potentiometer should be of that critical value to just stop oscillations at any capacity of the tuning condenser Ci, a condition that is achieved by adjusting resistance R. At this point the loop will operate as a receiver for radiophone speech or spark signals. However, when the push-button is depressed, the full potential is again applied to the tube and
AMPLIFIER TUBE
CONDENSER
RHEOSTAT
^VARIABLE CONDENSER PLAN
the set oscillates, i. e., transmits. When it is desired to establish communication between two such installations, the loops, in virtue of their directional transmission and reception, must be in the same plane, that is, pointing at each other. One station should be tuned (in receiving) to a wave at which there is no interference, and then, depressing the pushbutton, transmit and allow the other station to tune to resonance. Each station will now transmit and receive on the same wave. Boy Scout and similar organizations will doubtless find use for this set or those of like design.
By the addition of a set of the experimental type to his collection of props, the stage (or amateur) magician may add considerably to his repertoire of mind-reading mystifications. While it is obviously impossible to enumerate