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mate agreement with that commonly accepted. This shows that whatever effects may be produced in the lamps by frosting them, the mere absorption by the deposited carbon film, as explained above, is sufficient to account for a decrease in useful life of about 30 or 40 per cent.

Several weeks after this explanation was printed in the Electrical Review, Mr. Preston S. Millar published in the Electrical World* an account of some experiments made upon frosted lamps at the Electrical Testing Laboratories. It is very interesting to note that the results of Mr. Millar's experiments are in very good agreement with those made at the Bureau of Standards and further substantiate the theory outlined above. Quite recently Dr. A. E. Kennelly has put this theory into a mathematical form in a paper published in the Electrical World.

In the preliminary paper printed in the Electrical Review an experiment was outlined to determine if there are any other elements entering to bring about the short useful life of frosted lamps. This experiment has just been completed. Out of a number of 110-volt, 16-cp, 3.1-wpc, oval anchored-filament lamps five lots of 22 lamps each were selected and designated as Lots A, B, C, D and E. The nearest even voltage for each lamp was found corresponding to an initial specific consumption of 3.3 watts per mean horizontal candle. In all the subsequent photometric measurements the lamps were burned at these voltages. On the life rack, however, in order to hasten the completion of the investigation, the lamps were maintained at a voltage 12 volts higher than that at which they were measured on the photometer. This voltage corresponded to an initial specific consumption of approximately 2.3 watts per mean horizontal candle.

The five lots of lamps were measured carefully for initial mean horizontal candle-power, and then all the lamps except those of Lot A were placed on the life rack. The lamps of Lot B were burned for 9 hours, those of Lot C for 37 hours and those of Lots D and E for 78 hours, at which time it was expected that the lamps would have reached the 80-per cent point. On measuring all the lamps for mean horizontal candle-power, it was found, however, that the D and E lamps had only decreased in candlepower to 83 per cent of the initial value.

*Electrical World, April 20, 1907, p. 798.
+ Electrical World, May 18, 1907. p. 987.

All of the lamps with the exception of those of Lot E were then sent to the factory to be frosted. Upon their return they were all measured again for mean horizontal candle-power and placed upon the life rack, with the exception of those of Lot D, which already had burned for 78 hours, and had decreased. in candle-power (before being frosted) to 83 per cent of their average initial value. The lamps of each of the three lots, A, B and C, were burned the requisite number of hours to make the total time of burning 78 hours.

In this way all of the lamps were made to burn 78 hours, but the paths followed by the various lots were different. Thus, Lot A burned the total 78 hours after having been frosted; Lot B burned 9 hours while plain and 69 hours after having been frosted; Lot C burned 37 hours plain and 41 hours frosted, and Lot D burned the entire 78 hours before being frosted. The average candle-power of each lot at the various stages are given in Table V. In the first column are given the different

TABLE V

A COMPARATIVE STUDY OF THE CHANGES IN CANDLE-POWER OF PLAIN AND FROSTED LAMPS DURING LIFE

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tion coefficient.... o hrs. 9 hrs. 37 hrs. 78 hrs.

times at which photometric measurements were made, together with a statement of the condition of the lamps at that time, whether plain or frosted. In the next five columns are given. the average mean horizontal candle-powers of the five different lots at the various stages of their life. In the first line the actual

average candle-power of each lot is given, but in the succeeding lines the candle-powers are expressed in per cents of the initial value of each lot. The average candle-power of each lot determined immediately after the lamps were returned from being frosted is underscored, so that it is easy to see how many hours. each lot burned before and after having been frosted.

It is interesting to note the marked agreement among the different lots upon reaching the same point in the curve by different paths. Thus after 37 hours Lot A had decreased to 86 per cent, Lot B to 85 per cent, and Lot C (frosted) to 87 per cent, although the A lamps were frosted initially, the B lamps after 9 hours, and the C lamps after the entire 37 hours burning. Again, at the end of 78 hours the four lots A, B, C and D had decreased in candle-power to the same value within about one per cent, although the A lamps burned the entire 78 hours after frosting, whereas the D lamps were frosted at the end of the 78 hours, the other two lots having been frosted at intermediate points. In other words, these tests would seem to indicate that the frosting has no appreciable effect upon the life curve of the filament, the short, useful life of frosted lamps being due entirely to the increased absorption of the light by the deposited carbon film, as explained above.

Mr. Millar, in the paper referred to previously, described one set of experiments in which the candle-power of some frosted lamps that had burned a number of hours and were soiled increased by about 10 per cent upon being washed. One can readily see how the candle-power of frosted lamps could be changed considerably by the presence of dirt on the surface. In order to see whether this element played any part in the investigation at the Bureau, three A lamps that had burned 155 hours were measured for mean spherical candle-power before and after being washed. The increase in candle-power after the lamps were cleaned was about one per cent on the average. Since these three lamps had been exposed double the time used in the test, the effect of dust on the bulbs was evidently entirely negligible.

Upon completing the candle-power measurements after 78 hours, all of the frosted lamps, and, in addition, the lamps of Lot E, which had burned to 78 hours but had not been frosted, were placed on the life rack and kept burning, in order to see

whether there is any indication of an effect on frosting on the total life of the lamps. Unfortunately, up to the time of writing this paper the lamps have not all burned out. However, as there are only two lamps of Lot D left, the average life for each of the other four lots can be computed accurately. The two D lamps have already burned 310 hours, and may burn out at any moment. In giving the value of Lot D we shall assume that these two remaining lamps will burn out within the next few hours. For every 22 hours each one burns the average value will be increased one hour.

With this understanding in regard to the D lamps, the average total life for each group is as follows: Lot A, 104 hours; Lot B, 118 hours; Lot C, 133 hours; Lot D, 121 hours (?); Lot E, 118 hours. If frosting played an important part in determining the actual life of the filaments, the average total life of each successive group, in the order of the letters of the alphabet, would be greater than the preceding. There is no indication, however, of such a sequence. It is true that the life of the A lamps is the shortest, but the life of the E lamps, which should be the longest, is no greater than that of the B or D lamps, and is much less than that of the C lamps, which burned nearly three-fourths of their life after having been frosted. It is evident, therefore, that this test shows no marked effect of frosting on the life of the filament.

As shown in the last two columns of the table, the two extreme lots A and D were measured also for mean spherical candle-power. Here, as in the mean horizontal measurements, the average candle-power of the frosted lamps after 78 hours' burning is the same, independent of whether the lamps are frosted first and burned subsequently, or vice versa. The decrease in mean spherical candle-power, however, is slightly greater than the change in mean horizontal intensity. In discussing the change in absorption due to frosting the question was raised as to whether or not the absorption of new lamps is the same as that of seasoned lamps. The answer to this is evident from a consideration of Table V. The change in horizontal candle-power due to frosting is given in the last line of the table. The A lamps, which were frosted when new, showed an absorption of 2 per cent; the B lamps, frosted after 9 hours, an absorption of 4 per cent; the C lamps, after 37 hours, an absorption of 7 per cent; and the D lamps, after 78 hours, an

absorption of 12 per cent. The average value of 5.7 per cent, given earlier in the paper for seasoned lamps, is therefore probably several per cent higher than the correct value for new lamps.

Since, in seasoning the lamps that were used in studying the change in absorption due to frosting, no special care was exercised to see that they all burned at the same temperature for the same number of hours, it is not at all surprising that differences in absorption coefficient among different makes, or even among the individual lamps of any one make, occurred, since the carbon deposits may have been different.

Having shown that for the lamps studied at the Bureau and for the conditions under which the investigation was made, the shortened life of frosted lamps can be accounted for entirely by the increased absorption of life by the carbon film deposited on the inner side of the bulb, it is interesting to consider briefly the question of temperature effects due to frosting. Mr. Dickinson, of the Division of Thermometry of the Bureau, kindly undertook to determine the temperature of the bulbs of several of the A and D lamps at different periods of their life. By the use of a platinum resistance thermometer the figures given in Table VI

TABLE VI

A COMPARATIVE STUDY OF THE CHANGES IN TEMPERATURE OF THE BULBS OF PLAIN AND FROSTED LAMPS DURING LIFE

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