LEAD PEROXIDE DETECTOR.
The peroxide of lead detector makes use of no liquids, but still may be classed as an electrolytic since its action is of that nature.
Fig. 111. Peroxide of Lead Detector.
It consists of a pellet of lead peroxide held between an electrode of lead and one of platinum. Contrary to most other detectors, the resistance is increased upon the passage of electrical oscillations. The oscillations stimulate chemical action and increase a counter-electromotive force sufficiently so that a decrease in the current sent through the phones by the local battery takes place. The action may be outlined more in detail as follows. The current of the local battery decomposes part of the lead peroxide into its components, lead and oxygen. The lead ions are positively electrified and so they tend to pass upward toward the lead electrode which is negative. The negative ions of oxygen gas tend to pass downwards towards the platinum. But the lead and platinum electrodes with the intervening lead peroxide constitute a small cell acting independently of the local battery and sending a current in the opposite direction. This counter-electromotive force tends to send the ions in an opposite direction to that in which they are sent by the battery current. Upon the passage of electrical oscillations this counter electromotive force is increased and sufficient ions sent out in opposition to those of the battery current so that an appreciable drop in the current flowing through the telephone receivers takes place. The sudden current drop produces a sound in the receivers.
The lead pellets may be secured from a druggist who can mold them in his tablet press. They should be subjected to as great a pressure as possible in order to reduce resistance and prevent crumbling.
A piece of platinum foil about 1/2 inch square is placed beneath the pellet on the crystal electrode. A piece of clean, bright sheet lead 3/8 inch in diameter and 1/8 inch thick is laid on the pellet and the whole clamped together by tightening the thumbscrew passing through the collar on the spring, S. The detector is connected up similar to the "bare point," The platinum is made the positive of the local battery. Adjustment is secured by regulating the pressure.
It is very necessary that the pellets be kept dry, as otherwise a loud singing and hissing noise, due to the decomposition of the water, will render the reception of signals very difficult.