Mechanical Rectifiers.—By definition, a mechanical rectifier is a form of commutator operating in synchronism with the generator and commutating or rectifying the negative waves of the alternating current as shown graphically in figs. 2,076 and 2,078. The essential features of construction are shown in fig. 2,079.

Ques. Mention some application of a mechanical rectifier.

Ans. It is used on a compositely excited alternator as illustrated on page 1,192.

Electrolytic Rectifiers.—If two metals be placed in an electrolyte and then subjected to a definite difference of pressure, they will (under certain conditions) offer greater resistance to the passage of a current in one direction, than in the other direction. On account of this so called valve effect, electrolytic rectifiers are sometimes called "valves."

Ques. What metal is generally used for the cathode?

Ans. Aluminum.

Figs. 2,076 to 2,078.—Diagrams showing alternating currents, and partial and complete rectification.

Ques. What is generally used for the other electrode?