measurement, and an uncertain change in temperature resulted, in some cases probably amounting to a degree or more. Moreover, some of these measurements were made in testing the commutator and perfecting the method and were considered preliminary; more care would have been taken in ascertaining the temperature of the condenser if it had been supposed the results were to be of permanent value. We mean to make a new series of measurements under more favorable temperature conditions in the near future.

Plotting out the results for the four sections with temperatures and values of Cas abscissas and ordinates, respectively, we find a slightly different temperature coefficient for each section, ranging from 15 to 20 parts in 100,000 per degree. Reducing the values of all to 20° C. by means of these temperature coefficients, we have the values given in column 7 of the table. The average deviation of a single determination from the mean is 5.6 parts in 100,000 for the 0.5 mf section, 13.5 for each of the two 0.2 mf sections, and 33 for the 0.1 mf section. The last is excessive and may possibly be due to an actual change in the value, the last three determinations differing considerably from the first three. With the exception of the smallest section these mean deviations amount to less than the variation in the capacity for 0°.7 C. It is thus evident that if a series of determinations are to agree closely among themselves the temperature must be carefully determined. It is also evident that if the temperature is constant a good mica condenser is an instrument of precision and can be trusted to hold its value.

9. THE MEASUREMENT OF CAPACITY OF AIR CONDENSERS.

The first measurements made were on a so-called air leyden, purchased by the Bureau from Kelvin & Jas. White. The results are shown in the first part of Table VII. The correction capacity e was very large in this case, because the commutator was in an adjoining room from the bridge and condenser, and the connecting wires were several meters in length. In this determination three values of the resistance d were used (388,100; 386,000; 384,000 ohms) and the speed varied to balance the bridge in each case.

The two larger air condensers, No. 1 and No. 2, were designed by us and built in the instrument shop of the Bureau. We desired a sufficiently large air capacity to use as a standard in obtaining the constant of a ballistic galvanometer, which latter was to be employed in determining the capacity of mica condensers with relatively long charging periods. Each condenser has 200 nickel-aluminum plates, about 20 cm

TABLE VII.-Summary of determinations of capacities of air condensers.

GROVER.

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 e 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———5

OPTICAL PYROMETRY.

By C. W. WAIDNER and G. K. BURGESS.

INTRODUCTION.

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