Four examples.

The Time Thermometer is for use with the Watkins Thermo method of development. After diluting the developer to the correct amount according to the plate type and developer, the development time can be read directly from the height of the mercury. The scale is applicable for developers having a Temperature Coefficient of 1.9 and is marked: Pyro-Soda; Metol Quinol; Watkins Time Developer; Rodinal; Certinal; Azol and Victol. Versions for Rytol and Kodol developers and Autochrome plates were also advertised. Production ceased during World War I but recommenced around 1920.

With:

Scale: 40 - 80° F, 5 - 25° C. 8 ¼" length.

Scale: 40 - 90° F. 5" length in metal case. The case can be reversed to form a handle. 1940s.

Scale: 50 - 90° F. Alcohol. 5 ¾" length in metal case.

With:

Scale: 40 - 90° F. Alcohol. 5" length in metal case.

Notes:

With:

Small mercury thermometer attached to metal surround. Scale marked 60, 65 70 F. Kingsway address.

With:

Mercury thermometer 9" long, in polystyrene box.