Ans. Lead or polished steel.
Metals of low atomic weight exhibit the valve effect at high differences of pressure, and heavier metals at low differences of pressure.
Ques. Describe the "Nodon valve."
Ans. The cathode is of aluminum or aluminum alloy, and the other electrode, which has considerably more surface, is the containing vessel. The electrolyte is a neutral solution of ammonia phosphate.
Ques. Describe its action.
Ans. It is due to the formation of a film of normal hydroxide of aluminum, over the surface of the aluminum electrode. This film presents a very high resistance to the current when flowing in one direction but very little resistance, when flowing in the reverse direction.
Fig. 2,079.—Mechanical rectifier. The rectifier consists of two castings M and S with teeth which fit together as shown, being insulated so they do not come in contact with each other. Every alternate tooth, being of the same casting, is connected together, the same as though joined by a conducting wire. There are as many teeth as there are poles. The part M of the rectifier is connected to one of the collector rings by F, and the part S to the other ring by G.
Ques. What is the effect when a Nodon cell is supplied with alternating current?
Ans. Half of the wave will be suppressed and an intermittently pulsating current will result as shown in fig. 2,077.