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square, 0.064 cm thick, and 0.218 cm apart. They are mounted in a similar manner to the Kelvin “air leyden,” being thoroughly insulated on hard-rubber columns and inclosed in a cylindrical case.

In the measurements the terminal which is joined to the case of the condenser is always connected to the point D, the other terminal not connected to any exposed surface being joined to P. If these connections are reversed the measured capacity depends upon the surroundings of the condenser, increasing if a person approaches the condenser. The correction capacity c must be made with even greater care than for mica condensers, since it is relatively larger. In joining the condensers in parallel and in series the cases of the condensers are joined together, in the former case the two interior terminals being connected to P.

The capacity in series is one-fourth the capacity in parallel, since the two are nearly equal. The object of measuring them in parallel and in series was to check the determination with the values calculated from the separate determinations. The agreement is very satisfactory.

The increase in the capacity of these condensers between April 12 and May 21 was due to a change in their adjustment due to moving. They were not designed to be portable.

We are indebted to Dr. N. E. Dorsey, assistant physicist of the Bureau, for valuable assistance.

5834-No. 2–05





It is becoming generally recognized by engineers and technical men in charge of industrial processes carried out at high temperatures that it is usually necessary to measure and control the temperatures of these processes, and many instances might be cited where a change of less than 20° C. in the heat treatment radically alters the resulting product, and often such a small temperature change occurring unnoticed necessitates later the rejection of the completed product.

For a long time the problem of estimating high temperatures was dependent on the trained eye of the workman, but with the high degree of accuracy with which temperatures must be controlled to-day in many specialized lines of work, the requirements are such as can be fulfilled only by the use of a sensitive pyrometer. The two great advantages resulting from the use of the pyrometer, which are at once evident, are:

(1) Once the proper method of working a particular product has been found, this operation can be indefinitely repeated, thus rendering possible the exact duplication of products.

(2) The reproduction of any particular product is no longer locked up in the experience of a few workers, but becomes a matter of permanent record, which may be consulted at any time.

In this connection should be emphasized the advantage arising from the use of the same standard scale of temperature whatever type of pyrometer is employed, for this alone renders possible that important factor in the advance of scientific and technical knowledge—the interchange of experience among men.

There are many instances in practice where it is impossible to make use of any form of pyrometer which must be brought into contact with the substance operated upon, whether it be from the inaccessibility of

the object, its being in motion, or because the contact may be detrimental to the object or pyrometer. For all these cases and also where a rapid examination for the uniformity of temperature over a considerable area is required, some form of pyrometer entirely separated from the substance or furnace and thus acting at a distance is required, that is an optical or radiation pyrometer; and again, for the estimation of very high temperatures, such a pyrometer is the only form of instrument available.

There is an impression current that an optical pyrometer is of necessity a very delicate, mysterious piece of laboratory apparatus, not fitted for shop practice, and not to be trusted except in the hands of an expert, and even then giving results of uncertain reliability; but one of the primary objects of this paper is to show that there are several trustworthy optical pyrometers available, simple in operation, and suited to the most varied and exacting requirements of scientific laboratories and technical works.

In response to numerous inquiries which have been addressed to the Bureau of Standards on the availability and choice of pyrometric methods for particular problems, an experimental investigation of all the leading types of optical pyrometers obtainable has been carried out. This investigation bas also been stimulated by the great advances that have been made recently in the development of optical pyrometry, advances resulting in the production of several simple and trustworthy instruments, called into existence on the one hand by the pressing industrial need of them, and rendered possible on the other hand largely by the great progress made during the past ten years in our knowledge of the laws of radiation from incandescent bodies.

These questions will be treated under the following headings:
(1) General discussion of optical pyrometry;
(2) Laws of radiation:
(3) Methods of optical pyrometry;

(4) Description of instruments, including their calibration, range, sources of error, and precision;

(5) Comparison of various types of optical pyrometers; (6) Special problems in optical pyrometry.

It may be well to state at this point by way of explanation of the method of treatment adopted in this paper, that it has been the aim of the authors to discuss the subject from the point of view of its application primarily to industrial processes, and to answer those questions, that their observations in the shop and consultations with the experts in charge of these processes have shown nearly always arise when the applications of optical pyrometers are considered. Their experience

with these instruments has also strongly impressed them with the wide field of usefulness of these pyrometers in scientific laboratories for many lines of research.

A résumé of the most important work done in recent years on the laws of radiation has been added for the two-fold reason that it is the basis of the entire subject of radiation pyrometry and that this work has not hitherto been available to English readers.a

1. GENERAL DISCUSSION OF OPTICAL PYROMETRY. The temperature of bodies may be estimated from the radiant energy emitted, either in the form of visible light radiation or of the longer infra red waves that are studied by their thermal effects. For the estimation of temperature in this way use is made of the so-called laws of radiation. It would be beyond the scope of this paper to more than briefly outline the researches that have been made in recent years bearing on the laws of radiation. All that will be attempted here will be a statement of these laws, with a brief outline of the experimental evidence on which they are based, and the way in which they have been applied to give an idea of temperatures beyond the range of all ordinary pyrometers that have to be exposed to the temperatures to be measured, e. g., the temperatures of the filament of an incandescent lamp, the electric are, the electric furnace, and the boiling points of metals.

A number of excellent pyrometers have been introduced into practice that are based on the photometric measurement of the intensity of the light emitted by incandescent bodies.

Most of these pyrometers measure photometrically the intensity of the red radiation. This is done for two reasons, first, because the color of the light from the incandescent source will undergo wide variations as the temperature changes and it will thus be difficult to compare it with the light from some standard source, so that by passing the radiation from both sources through a red glass (or prism) the photometry is reduced to the comparison of two lights of the same color; and secondly, the use of the red radiation enables the measurements to be carried down to lower temperatures, as red light is the first to become visible.

When we consider the enormous increase in the intensity of the light with rise in temperature, this method appears especially well adapted to the measurement of high temperatures. Thus, for example, if the intensity of the red light 1=0.656u, emitted by a body at

a Since the beginning of this work an excellent discussion of the laws of radiation by A. L. Day and C. E. Van Orstrand has appeared in the Astrophys. J., 19, p. 1; 1904.

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