restrainer

One who or that which restrains; specifically, in photography, a chemical which is added to the developer for the purpose of retaṙding its action, especially in the case of an over-exposed plate, or in order to obtain greater contrast or intensity in a naturally weak plate. Acids, sodium sulphite, bromides, and other substances act as restrainers.

Specifically, in electricity, a substance used in the manufacture of the electrodes of magnetite arc-lamps the function of which is to reduce the rate at which the terminals are consumed.

One who, or that which, restrains.

Something that restrains

A chemical compound (such as potassium bromide), added to a developer to prevent fogging

a chemical that is added to a photographic developer in order to retard development and reduce the amount of fog on a film

a person who directs and restrains