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DRY PLATES, FILMS AND PAPERS 187
His invention is based upon a discovery he made that the table of times and temperatures calculated by the formulae due to Ferguson, Houdaille, and others, could be accurately represented by a logarithmic scale of figures representing time placed in contact with an even division scale of temperatures.
The instrument is known as the Time Thermometer, and it is used for indicating directly the time required to complete development for any one group of developers. It is like an ordinary chemical thermometer in appearance, but no temperatures are marked upon the scale ; the height the mercury rises in the tube indicates direct the time for development. Of course the expansion of the logarithmic time scale has been carefully adjusted by the makers to fit the particular thermometer to the temperature coefficient of the developer used. The pattern on the market has a fixed scale, and is devised for that group of developers having a temperature coefficient I'd about, and it can also be used for the Kodak tank developer.
One side is marked for dish development with ordinary dilution, the time being 6J minutes at 60° F. The other side of the mercury tube is marked for tank development with diluted developer, and is set for 24 minutes at 60° F.
Watkins' idea is that the thermo-developers should be issued so as to require 6 J minutes at 60° F., different dilutions of the developer to be then indicated by certain code letters given to the plates. Thus he classes the plates by the code letters :
VQ. Q. MQ. M. MS. S. VS. 1 H If 21 3 4 5
and the figures under are the drams of Watkins' time developer required in 3 ozs. of water for dish, or 10 ozs. for tank development.