DEVELOPMENT, THERMO A name invented by Alfred Watkins for a system of developing in which the duration of development is varied as the temperature of the developer varies, to distinguish it from " time development " pure and simple. Although it had been known for a long time that develop ment was prolonged in cold solutions, Houdaille in 1903 was one of the first to suggest a definite rule, in conjunction with hydroquinone, to the effect that a variation in temperature of the developing solutions of 1° C. caused a variation of 5 per cent. in the time of appearance of the image and the duration of development. In 19o5 Ferguson and Howard published a method of obtaining a given gamma or degree of contrast with a developer of constant composition at varying temperatures by developing strips of a plate exposed behind a sector wheel for different times, measuring the gammas obtained and plot ting them on a chart. Later, Ferguson sug gested a simpler method, in which only two strips were developed for given times and the tempera ture coefficient of the developer found by a very simple logarithmic calculation. The objection to both these methods is that a sector wheel and photometer are required. As, however, Ferguson's method is simple, and the basis of that to be described later, it is briefly given here.
The first thing to determine is what is known as the temperature coefficient of the developer used, or the increase in velocity of development for a rise of in temperature, from which the increase in rapidity of development for r° can be found by dividing the logarithm of the temperature coefficient by io. The usual mathematical expression for the temperature coefficient is— velocity at (t io°) C.
bro ; velocity at t° C.
therefore b, or the increase for I°, is— log. of temperature coefficient io = log. b.
Ferguson has suggested that two strips of a plate should be exposed to the same graduated series of light and one developed at t° C. and the other at t° x° C. ; then the times required to obtain the same gamma or degree of contrast on each plate will be M and m, and as the times are inversely proportional to the velocities to b = log. M — log. m g.
in which b = bhe temperature coefficient for r° C., and ro log. b will be the temperature coefficient for io° C.
To make this quite clear two strips of a plate should be developed so as to show the same degree of contrast, one at a given temperature and the other at this temperature plus a certain number of degrees. Two strips A and B of a plate were exposed to the same graded series of light, and A took 3i minutes to obtain a gamma = I at io° C. and B took 2i minutes at z8° C., then— log. 3.5 = log. 2.5 = •3979 '1462 Now the difference of temperature (A as it is usually written) was 18 — io = 8°
. • . •1462 4- 8 = •o182 = log. b . • . zo log. b = •o182 x io = •182 = the temper ature coefficient for the plate and developer used. Having found the above, to find the necessary duration of development at z 4° C. to obtain the same gamma the formula is— log. M — x log. b = log. m or, in words, from the logarithm of the time required at zo° C. subtract 14 — ro = 4 times log. b, and the result will be the logarithm of the time required ; assuming that log. b = •0182 as above, then— log. 3.5 = 4 log. b= •0182 x 4 = -0728 = log, of 2.96 minutes, the time required.
Watkins has done away with the sector wheel or graded series of lights and the finding of the gammas ; he merely exposes a plate on a landscape including some sky and cuts it in two, or makes two exposures. It is essential to have some means for warming up the developer, dish, and measure, and also to have a thermome ter. When the developer, dish and measure are warmed up, say to P. (about 24° C.) the plate is placed in the dish and flooded with the developer, the time accurately noted, and the first appearance of any image also noted ; the plate being now of no further use it may be thrown away. The second half of the plate is now developed in a developer of exactly similar composition, only colder, the time of appearance noted, and we have all the factors necessary for finding the time of development for any tempera ture with that particular plate and developer if we know also the factorial number (see "Devel opment, Factorial ") of that developer. Sup pose, for example, that a metol-hydroquinone developer with a factor of 15 is being used, and it is found that at 50° F. (lo° C.) the first appearance of the image takes place in 40 seconds and at 66° F. (i8.8° C.), the first appear ance takes place in 28 seconds ; then turning to a table of logarithms we find that log. 40 = 60206o and log. = 1.447158, then 1.602060 — 1.447 158 = o•1549o2• Now the difference in temperature is 66 — so = 16, then 0•154902 5- 16 -= •oo9681 = log. of 28.3 seconds, and as the factor for this developer was is, then 28.3 x 15 = 424 seconds = 7 minutes practically, which is the time required to develop the plate at 57° P. (i4° C.).
It is obvious from this that we can calculate a table for every degree (or two degrees will be enough) rise or fall in temperature of the developer, by multiplying the log, factor by 2 and adding for every two degrees drop or sub tracting in the case of a rise, and then multiply ing the number by the factorial number. This may seem somewhat complicated, but the logarithms have merely to be read from a mathe matical table-book and simple division, multi plication, and subtraction performed.