Richardson's handbook of projection (1930)

1040 HANDBOOK OF PROJECTION FOR of T2 as it did before; and we see that our rectifier is now using both halves of the A. C. wave. We must be careful to point out that the process just described does not produce true D..C, but a hybrid current called "rectified A. C," Fig. 391, which resembles D. C. in that it always flows in one direction, but differs from D. C. in that it is not steady. Every half cycle it drops to zero, then shoots up to a maximum, drops to zero again and so on. In other words, it is a pulsating current. The reason for this is that, as shown in the diagram, Fig. 391, the A. C. does not suddenly change its direction at the end of each wave, but drops down gradually to zero and then increases again in the opposite direction to the maximum for the next half wave, just as when you are driving your car forwards you cannot start to go backwards in an instant ; you must slow down, stop, and then build up reverse speed. For some purposes, such as charging batteries, there is no objection to rectified A. C. with its ups and downs — it may even be sufficient to use half wave rectification, thereby * eliminating the second tube. However, for amplifier plate supply we must have true D. C, even more free from irregularities than the current obtained from the mains in most D. C. districts. Otherwise the output of the amplifier would be rendered useless by reason of the loud hum accompanying it. To obtain from our tubes the kind of current we want for amplifier work, one further step is necessary. The rectified A. C. must be made smooth and steady by passing it through the filter previously mentioned, which is a combination of choke coils and condensers that may be described as an electrical shock absorber. In your car the road bumps are prevented from reaching you