circuit the burner takes 0.5 ampere at 215 volts, and the resistance 0.5 ampere at 20 volts, while one of the smaller lamps for use on the same circuit takes 0.25 ampere at 215 volts and 0.25 ampere at 20 volts for the burner and resistance respectively. The burner and heater are very fragile, and should never be handled except by the porcelain plate to which they are attached. The lamps burn in air and emit a brilliant white light of high actinic power, the intrinsic brilliancy (c.p./square inch) varying from 1000 to 2500, as compared with 1000 to 1200 for ordinary metal filament lamps, and 300 to 500 for carbon filament lamps.
The chief advantage of the Nernst lamp from a photographic point of view lies in the fact that it produces abundantly the blue and violet rays which have the greatest chemical effect upon a photographic plate or film. These rays are known as chemical or actinic rays, and are only slightly produced in some types of incandescent electric lamps. Carbon-filament lamps are very poor in this respect.
Because a light is visually brilliant it must by no means be assumed that it is the best to use for purposes of photography, and this is a point over which many photographers stumble when using artificial light. Many sources of light, while excellent for illumination, have very low actinic powers, while others may have low illuminating but high
actinic powers. A lamp giving a light yellowish in colour has usually low actinic power, while all those lamps giving a soft white light are generally found to be highly actinic.
In addition to the actinic value of the source of illumination, the photographic film used must be very carefully chosen, as the chemical inertia of the sensitised film plays an important part in the successful reproduction of the picture, and also, to a certain extent, affects the speed of transmission. The length of exposure, the amount of light admitted to the film, and the characteristics of the film itself, are all factors which have a decided bearing upon the quality of the results obtained, and the film found to be most suitable in one case will perhaps give very unsatisfactory results in another.
In photo-telegraphy the length of exposure is determined by the time taken by the transmitting stylus to trace over a conducting strip on the metal print, and this time, of course, varies with the density of the image and also with the speed of transmission.
The film in ordinary photography is chosen with regard to the subject and the existing light conditions, and the amount of light admitted to the film and the length of exposure are regulated accordingly. No such latitude is, however, possible in photo-telegraphy. With each set of apparatus
the various factors, such as the light value, the amount of light admitted to the film, and the length of exposure, will be practically fixed quantities, and the film that will give the most satisfactory results under these fixed conditions can only be found by the rough-and-ready method of "trial and error."
The films in common use are manufactured in four qualities, namely, ordinary, studio, rapid, and extra rapid. These terms should really relate to the light sensitiveness of the film (or, as it is technically termed, the speed), but at the best they are a rough and very unsatisfactory guide, for the reason that some unscrupulous makers, purely for business purposes, do not hesitate to label their films and plates as slow, rapid, etc., without troubling to make any tests for correct classification.
The speed of photographic films and plates is generally indicated by a number, and the system of standardisation adopted by the majority of makers in this country is that originated by Messrs. Hurter & Driffield, abbreviated H. & D. In their system the speed of the film and the exposure varies in geometrical proportion, a film marked H. & D. 50 requiring double the exposure of one marked H. & D. 100. The highest number always denotes the highest speed, and the exposure varies inversely with the speed.