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temperature of this source was 1625 abs., the different instruments agreeing to within 3° in this determination. Each of the instruments was also calibrated over a considerable temperature range in terms of a thermocouple using a Lummer-Kurlbaum black body as source, thus obtaining a whole series of comparison points. The two methods gave concordant results.

The values of absorption factors (K) vary with the wave lengths of the light used, K’ decreasing quite rapidly for shorter wave lengths. Thus a given absorbing glass gave K=1500 for 1=0.6514, and only 360 for 1=0.550u. Also with these glasses the value of K for n glasses is not K", as is generally assumed, but something less than this quantity. Four glasses of the same quality and thickness gave, for 1=0.6514, the following results, as determined from a large number of observations, using all three pyrometers in this determination:

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The results of our determination of the arc temperature for a current of 15 amperes are given in the three following tables. The carbons were exceptionally homogeneous, prepared especially for optical projections and arcing very readily. The positive carbon (13 mm diameter) was mounted horizontally and the negative (11 mm diameter) vertically. The potential across the arc was about 65 volts. In order to prevent the formation of a deep crater on the under side of the positive carbon, the latter was occasionally trimmed down so that the hottest part of the crater could be readily viewed in a horizontal line.

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Although the three pyrometers give practically an identical value (3700° abs.) for the arc temperature, yet we do not consider all the observations of equal weight. The red glass 1=0.630j is not sufficiently homogeneous for good pbotometric measurements; with the Holborn-Kurlbaum instrument, for instance, the filament of the lamp can not be made to disappear when using this glass. Glass 1 0.6514, on the other hand, leaves practically nothing to be desired as to its homogeneity, and the results obtained with it have the greatest certainty. The values of the equivalent wave lengths for the red glasses were verified by Dr. Nutting to an accuracy well within the limits of error from other sources.

The relative temperatures obtained with the various instruments are independent of the absorption factors, the latter being dependent upon the absorption glasses, which were the same for all the pyrometers, and upon the wave length used. The absolute values we have obtained for the arc temperatures would evidently be in error if the values of the absorption coefficients are incorrect. The values chosen are the best that could be deduced from several hundred observations made with all the pyrometers. The absorption factor for green light (1=0.550u) is, however, less well determined.

As to the pyrometers themselves and the reliance to be placed upon the results obtained with the different instruments, the greatest uncertainty seems to be with the Wanner, because it is more difficult with this instrument than with the others to be sure that one is sighting upon the desired spot, as the instrument is not a telescope; also, the extrapolation of the scale of this particular instrument is somewhat uncertain. As between the determinations with the Le Chatelier and Holborn-Kurlbaum instruments, the preference is in favor of the latter, as a smaller area is used in the photometry and there are more independent checks upon its calibration; thus, using three comparison lamps, identical results are obtained.

On the whole, a better agreement among different instruments employing such varied methods of measurement could hardly be expected at such a high temperature. Our experiments with pure graphite have shown that the value, 37000 abs., would not be increased by more than 50°. The effect of material of carbons and of current density will also be considered.

VARIATIONS IN TEMPERATURE OF THE ARC.

(a) With current. It is generally accepted, since Violle so stated, that the temperature of the arc is independent of the current, and this temperature is assumed to be the boiling point of carbon. There are, however, at least two reasons for which we should expect this temperature to vary with the current. As the current is increased there will be a tendency to superheat the viscous carbon layer from which the vapor boils, even though this vapor does not have a higher temperature than the normal boiling point, and as it is this viscous layer which is observed, variations of current above a certain limit should be accompanied by changes in temperature. Again, with low current a smaller area is heated, so that it will be more cooled by conduction.

An examination of the observations from which it has been inferred that the arc temperature is independent of current seems to indicate that this conclusion is unwarranted, for no observer has published a series of results sufficiently concordant in themselves or of sufficient number to enable him to state with certainty whether or not the effect exists. This has been due largely to the tediousness or inadequacy of the methods employed.

The constancy of brightness was first announced as probable by Rossetti in 1878, and Violle was the first to state as a result of his own experiments that "the brightness of the positive carbon is rigorously independent of the electric power expended to produce the arc, changing from within the limits 10 amperes at 500 watts to 400 amperes at 34000 watts.” Carbons of 3.5 cm diameter were used for the high currents, but no details of observations are published nor the precision of his spectrophotometric method, which is certainly sensitive, though it would be expected that any effect for very high currents would be neutralized, in part at least, by the larger carbons used. Furthermore, results with carbons of varying size and quality are not comparable. Although Violle showed undoubtedly that with carbons of different sizes adapted to carrying currents of from 10 to 400 amperes, there is no considerable change in temperature, yet it would seem that he did not show conclusively that for a given sized carbon there is no variation of temperature with current for the brightest portion of the positive crater.

Wilson and Gray found that changing from 14 to 25 amperes “the temperature then appeared to be a little higher than with the smaller current. Later experiments

indicate an exact

*

*

a Violle: J. de Phys. (3), 2, p. 545; 1893.

equality of temperature.” They do not give any data, but their published observations on the arc temperature indicate that it would be difficult to detect variations by their method.

Wanner states that “changing the current one and a half times (15 to 22 amperes) remains without influence.” Wanner's published results for a given kind of carbon are few in number (four) and vary over 90° under the same conditions, so that any differences due to current changes might be masked.

Our experiments on the variation of arc temperature with current were first made with a Holborn-Kurlbaum pyrometer, which is peculiarly well adapted for these measurements, as it is readily sighted, and thus the wandering of the brightest spot on the crater can be easily followed. A very small photometric area is employed, and observations may be taken rapidly within intervals of a few seconds.

In order to secure a good-sized image of the crater, so as to facilitate the photometric measurements, the instrument, provided with auxiliary lenses and suitable absorption glasses, was placed as near as possible (12 to 25 cm) to the arc, which was mounted as previously described. The current from a 120 volt storage battery was varied without the observer being aware of the actual changes, and to render the settings of the pyrometer as unbiased as possible an additional rheostat in its lamp circuit was also worked independently of the observer. No observations were taken on a hissing or humming arc, nor until the arc burned normally after changing current, and the conditions of constant length of arc and constant P. D. across the arc were very closely maintained, the latter being measured with a Weston voltmeter; the value of the P. D. was about 65 volts. Two observers obtained practically identical results, which are here summarized.

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