570 HANDBOOK OF PHOTOGRAPHY 



perature, the water in the larger tray may be held continuously at the required tem- 

 perature above or below ambient. 



If cold water is not available in the summer, or warm water in the winter, recourse 

 must be had to ice, or to water heaters. Ice should not be placed in the solution itself 

 because it will dilute the solution as it melts. It must be realized that it will be diffi- 

 cult to maintain a tray or tank of developer at a desired temperature by the use of ice 

 packed around the solution, and that only a cooling effect may be secured. The 

 temperature of melting ice is 32°F., and a tray floated on melting ice will tend to attain 

 this temperature. 



To heat trays or tanks of solution, electricity may be used. A tray may be placed 

 over a metal-topped box (or a portion of the workbench with a cut-away portion over 

 which is placed a metal plate or sheet). In the box, or below the cut-away portion of 

 the bench, ma}^ be placed electric lamps which are turned off or on as the conditions 

 require. They may be controlled automatically by means of a thermostat placed in 

 the solution to be maintained at a given temperature. Cheap bimetallic strips can 

 be purchased and often serve very well for this temperature-maintenance 

 purpose. 



It is easier to heat than to cool a solution. Therefore it is a good plan to surround 

 a tray with melting ice which is raised in temperature by electricity. (See data on 

 floats in paragraph on Sinks above.) 



Timing Photographic Processes. — Many timing devices, mostly clocks, are on the 

 market. Equipped with a large second hand, they enable the photographer to time 

 accurately any process not taking place in absolute darkness. Some timers are 

 equipped with an alarm which warns the worker when a certain time has elapsed. 

 These are especialh^ useful when the worker is processing panchromatic materials in 

 darkness. 



If exposures of only a few seconds are required to be made in darkness, e.g., in 

 making color-separation negatives from color transparencies, a useful device is one 

 which automatically turns off the exposing light at the end of a certain period. These 

 devices may be based on one of several principles. 



A synchronous motor maj^ revolve a contactor through a series of cog wheels. In 

 general illumination the contacts may be moved to such a position that the exposure 

 will take place for a certain number of seconds; then the sensitive material is placed 

 in position in darkness, and a switch is thrown which will start the motor and at the 

 same time turn on the exposing lamp. When the motor has rotated the contact 

 wheel a certain number of degrees, contact with the light source is broken, and the 

 exposure is ended. 



Other devices are electronic in nature, using vacuum tubes. In this case the funda- 

 mental principle is the charge and discharge of a condenser which is in the grid circuit 

 of the tube. The rate at which a condenser charges (or discharges) is a function of 

 the resistance in series or in parallel with it. A certain combination of resistance 

 and capacity may be chosen which will keep the grid of the tube overbiased for a cer- 

 tain period of time. At the end of this time the charge in the condenser is dissipated, 

 and the grid draws current which is reflected in a current in the plate circuit of the tube. 

 In this circuit is an electrical relay whose contacts control the illumination by which 

 the exposure is made. These electronic devices maj' be made to hold a circuit closed 

 (or open) from a fraction of a second up to 1 min. or more. In general they are less 

 stable and accurate over a period of time than a purelj^ mechanical contactor timer as 

 represented by a synchronous motor. 



The cheaper synchronous-motor units have a tendency to stall. A loud buzz 

 indicates that the motor has stopped (or a relay in the unit has struck or is 

 chattering), with the result that the exposing light does not turn off. 



