Page:Popular Science Monthly Volume 91.djvu/468

This page needs to be proofread.


452

��Popular Science Monthly

��Alternating current is supplied by the generator G to the stator S of the Gold- schmidt alternator through the choke-coils A and the interrupting key K, as shown in Fig. 2. As the alternator is revolved by an auxiliary motor, the current set up in the rotor R is made very large by connecting the circuit DEF to the rotor so that the impedance, that is, the apparent resistance, to the current is made very small and the current made correspondingly

���One of the two frequency-doubling trans- formers of the Arco-Joly sending system

large. The current is then reflected back into the stator as explained before, and the new current of double the original fre- quency is also provided with the tuned electrical path CL so that the impedance for it is also very small. Reflecting the current again into the rotor gives an- other current of three times the original frequency. This too is provided with a circuit of very small impedance, which allows the current to become comparatively large. The last reflection takes place from the rotor to the stator and it is this current that was used in the antenna. To accom- plish this, the antenna was connected to / and the ground was connected to /, as shown in the diagram; the loading coil B and the condenser C having been adjusted until the antenna was tuned to agree with the last reflected frequency.

In this manner, currents which have the high frequency of 60,000 — or 60,000 cycles of changes in direction per second — were readily obtained. The alternator was driven at a speed of nearly 10,000 revolu- tions per minute. The best results were found to be obtained on reflecting the cur- rents four successive times. Theoretically, there is no limit to the number of times they may be reflected, but practically on reflect- ing them more than four times, the electrical and magnetic losses become excessive.

��The Arco-Joly system uses a number of separate transformers so constructed that the frequency of the initial current is doubled every time it goes through a transformer. The construction of the separate transformers is as shown in Fig. 3. The transformer has two independent cores and an auxiliary circuit K, K' supplying a direct current, which is sufficiently large nearly to saturate both of them magnetical- ly. The initial current is supplied to the primaries P and P' of the transformers by the alternator G and the curve represent- ing its strength is an ordinary sine curve as shown by A BCD in Fig. 4. The capacity C and the inductance L are made of such values that the impedance of the circuit CLPP is very small. This causes the current in that circuit to become correspondingly large. On account of the manner in which the auxiliary winding K is connected, as the initial current passes through the stage shown by AB in Fig. 4, the increased magnetization of core N amounts to practically nothing. This is because the core is already saturated by the auxiliary direct current and also because the winding of P is such as to make it tend to add to the already saturated flux. In N', however, due to the fact that winding K' is in the reversed direction to that of K the current in P' tends to decrease the total flux in the core N'. This can readily be done, and as the flux diminishes in strength, the change in flux induces an electromotive force in

���Curve showing how the first transformer ■ makes the original current uni-directional

the secondary winding S'. The result is the secondary current roughly represented by EF in Fig. 4. As the initial current reverses in direction, as shown by CD in the same illustration, it is evident that the phenomena in the core N- and N' are

�� �