360 340 320 300 280 260 240 230 220 Fig. 5.--Graphs showing the Relationship between the Lower Critical Voltage and the Capacity, for a Fixed Charging Voltage E. GRAPH A'.-Large Capacities (From 5μF maximum). Pressure 0.75 mm. Hg. Graph is typical, similar curves being obtained for pressure range 3 mm. to 0.2 mm. GRAPH A'.-Large capacity scale (Maximum 0.25μF). Same results E = 580 volts. RaBr ionizing agent. Air. p = (16·65 × 0·01) cm. = 1·665 mm. Similar relations were found to hold for the molybdenumwire electrode discharge-tube (see graphs of fig. 6). The conductivity of the tube is, however, much less than the conductivities of the other tubes, because the area of the cathode is very small. The capacity range of abnormal cathode fall extends consequently much further into the region of small capacities. Indeed, for capacities greater than some 0-2 microfarad, the actual time duration of the discharge becomes considerable and the conditions for accurate performance of the present method break down. (The time of "flash" becomes comparable with the time period of the ballistic galvanometer employed, and an initial "kick," always in the same direction, inhibits results (see Taylor and Stephenson, Journ. Scien. Instr. loc. cit.).) It is observable that V' in the case of this tube diminishes slowly with the capacity down to about 0004 microfarad, when rapid diminution with the capacity ensues. In previous experiments on the variation of the critical resistance for 350 330 310 290 270 250 230 "flashing" (R) for this particular tube (see Taylor and Clarkson, Phil. Mag. loc. cit.) it was found that the critical resistance R, decreased slowly and almost linearly with the capacity down to a capacity of 0.004 microfarad, when rapid diminution of Re with further capacity reduction set in. The variation of the critical resistance is thus intimately connected with the variation of Vo'. Fig. 6.-Graphs showing the Relationship between the Lower Critical Voltage and the Capacity, for a Fixed Value of the Charging Voltage E. Air discharge-tube. Parallel molybdenum-wire electrodes tube. Charging Voltage E, 720 volts throughout. GRAPH A.-Pressure 2.54 mm. Small capacity scale (maximum 0.055F). GRAPH B.-Pressure 1.72 mm. GRAPH C.-Pressure 1.13 mm. GRAPH D.-Results for 1:72 mm. (graph B), but with enlarged capacity scale (0·02 μF.). GRAPH D'.-Results for 1·13 mm. (graph C); enlarged capacity scale. We are indebted to Prof. G. W. Todd, of Armstrong College, under whose supervision the experiments were carried out, and to the Department of Scientific and Industrial Research for the grant which has enabled one of us to undertake the work. 1 CXI. The Use of Commercial Plates in Research on the Latent TE loga/(a-x) (which we will call n), the light intensity I, and the time of exposure t. It is unnecessary to discuss their result in detail here; the main conclusion from their work was that the relation between n and I was expressed by an equation completely different in type from that which connected n and t. The results of much later work in the British Photographic Research Association laboratories has not confirmed this wide difference, and only partially confirmed the actual relationships which were found. Further, it has been found that a relationship which holds for one emulsion is not necessarily exactly the same for another, and this is the main difficulty in the way of establishing exact quantitative relations between the photographic effect and the conditions of exposure. Since Sade and Higson's paper was written, it has been realised that the quantity we have called n is of great significance, in that it is actually the numerical measure of the number of reduction centres (expressed as average number per grain) which are formed by the light. This has ied to many laborious experiments on the relation between n and the exposure, and for many emulsions it has now been shown that the curve connecting n and E (E=Ixt, where I or is variable), for monochromatic light and *Communicated by Dr. T. Slater Price, O.B.E., D.Sc., F.R.S. |