The total number of 15 watt lamps tested was 24, one half of which was obtained directly from the manufacturer and the other half bought in open market. It is well to mention at this time that this may have been the cause of the different qualities as brought out by the life tests.

The lamps were rated at 1.31 watts per horizontal candle power and were supposed to have a useful life of 1000 hours. The voltage ratings of those obtained from the factory were 114-112-110 and those bought in open market were 115-113-111. The correct efficiency of the lamps as found by test was 1.34 watts per candle power. A shot diagram follows which shows the actual rating of the lamps at high efficiency.

All readings were made by a Lummer-Brodhun photometer and the voltmeters and ammeters used were carefully standardized. The ammeter was placed beyond the voltmeter in order to get the true current taken by the lamp. The drop across the ammeter was taken into account in the voltmeter readings.

Life tests were made under two conditions, namely, a shock test where the lamps received severe vibrations and a test under ideal conditions, i.e. no jar and constant voltage. In order to obtain vibrations for the lamps upon the shock test, a small motor, with its shaft pulley off set, was screwed rigidly to a table. The lamps were placed in a normal position upon the table by means of wooden frames. The result was that when the motor was running it had a pounding effect, thus putting the table, consequently the lamps, in a state of severe vibration. The filaments of the lamps could be seen violently shaking for some distance. The test was indeed a hard one, and one that would not be found in many actual cases. It is very doubtful if railway lamps are subjected to such a strain and they are of the heavy filament low voltage type. The following photograph shows the arrangement above described. Ten 15 watt lamps were used on this test, the remainder shown being 20 and 25 watt and carbons.