Fig. 8. Tuned Hydraulic Transmitter and Receptor.
Tuning is represented graphically in Fig. 8. The two floats A and B are not only resting on the surface of a pool of water as in Fig. 3 but are also suspended from the springs S and S'. The springs will have, like a pendulum, a definite time of rising and falling, or period of oscillation, depending upon their length. If we strike the float A the spring will cause the float to rise and fall at a definite rate and send out a little wave or ripple with every oscillation. If the springs S and S' are of the same length, the float B will be caused to oscillate with every wave sent out by A, for, the periods of the springs being equal, B will be permitted to rise with a wave and fall again just in time to be raised by the next oncoming ripple. On the other hand, if the springs are of different lengths, B may only rise slightly and in falling meet an oncoming wave which will cause it to rise before it has reached its lowest point and so dampen or weaken its oscillations that they either do not become very strong or are entirely obliterated. Thus several floats having different periods of oscillation might be sending out ripples in the same pool, and the float B could be made to respond to any of them by adjusting the length of the spring.
We may also see in this illustration the part that tuning plays in causing the apparatus to emit or receive more powerful impulses. When the rope in the untuned apparatus illustrated in Fig. 3 is jerked, the block A oscillates only once or twice before a new jerk is required to keep it in motion. In Fig. 8 it is quite the contrary, for when an impulse has been given to the float A it will oscillate much longer than the untuned float before it requires to be set in motion again. Likewise the float B in Fig. 8 will oscillate longer and more powerfully than the float B in Fig. 3, when once it has been set in motion.
Fig. 9. Tuned Wireless Telegraph Transmitter and Receptor
Fig. 9 shows a diagram of a simple wireless telegraph system employing an inductance and capacity for tuning the circuits. When the induction coil is in operation it charges a condenser. The condenser discharges through the sending helix and across the spark gap. The sending helix is merely a spiral coil of wire of large diameter, and constitutes the greater part of the inductance in the circuit. Two movable contacts, A and B, make connections with the helix. The spark gap, condenser and lower portion of the helix up to the movable contact A are known as the closed circuit. By shifting A, more or less inductance may be included in the closed circuit until resonance is secured. The aerial, the inductance from the contact B down, the condenser and the ground compose the open circuit. By varying the contact B more or less inductance may be included in the open circuit and its period altered until the oscillatory currents of both circuits flow in the same period of time. The closed and open portions of the transmitting helix form an auto transformer, and the voltages of the open circuit are raised above those of the closed circuit.
The tuned receptor shown in Fig. 9 is the simplest form possible and is known as the single slide system. The tuning coil or helix is much longer in proportion to its diameter than the sending helix, and is made of finer wire, since it does not carry such heavy currents. When the contact is slid up or down on the tuning coil, the inductance of the circuit is varied. Since the oscillating currents in the receiving aerial have the same frequency as those in the radiating aerial, the receptor must have the same relative values of inductance and capacity. This condition is obtained by varying the slider until the signals in the telephone receivers are the loudest.
In practice more than one sliding contact is used, and these together with adjustable condensers make the circuit more complicated. These devices are necessary because oscillations may be forced on a receptor by a near-by transmitter unless other precautions than the "single slider" are taken. Such circuits are illustrated in Plates IV and V. With them it is possible to obtain a considerable degree of selectivity and "tune out" an undesirable message.