First, when a and b are both at the freezing point, we make the distances e f and e h equal, and measure the difference between distances e g and e k. Second, when a is at any desired temperature and b still at the freezing point, we make distances e f and e h again equal and again measure the difference between distances e g and e k. So we measure the expansion of bar a by a scale absolutely unaffected by temperature.
Another instance: The question how much a cubic inch of water weighs is one on which scientific men of different countries are not agreed. Different determinations of the weight of a cubic decimeter of water differed by 480 milligrammes or about one-twentieth of one per cent. The reason was not inaccuracy in weighing, but the fact that the dimensions of the body weighed in water cannot be accurately determined, but by our method we can now measure the dimensions of the body to be weighed in water, and may hope to better the determination of the relation between standards of length and of weight.
Mr. Eisenmann; How are the differences of the temperatures of the bars and their supports overcome? Are they assumed to be of the same temperature.
Prof. Morley: We assume nothing. When our two cold bars have remained motionless for some time, we measure their difference in length. If we change the temperature we wait till they become motionless again, and again measure the difference. We do not even assume equilibrium. We verify the fact that the bars are motionless on their supports by examination.
Mr. Eisenmann: When I was engaged upon the old method, defining some standards, we had sweat boxes in which the temperature differed several degrees. We were not allowed to remain in the chamber more than 15 minutes.
Prof. Michelson: In the case of all comparisons made heretofore, the difference between what the thing rested upon and the thing itself had to be taken into consideration. In our experiments we do not have to consider that.
Mr. Eisenmann: After you have measured the distance from c to d in Fig. 2, and determined the number of wave lengths, is that affected by the temperature? How long must it have been submitted to a constant temperature?
Prof. Morley: We like to have our standard in ice for 24 hours. But we assume nothing. We prove the length c lo d consists of a counted whole number of wave lengths and a fraction. The fraction can be measured in a very simple way by measuring with a micrometer the diameter of the interference fringe produced in sodium light. When this fraction remains constant, the length of the bar is constant. The observation required is as simple as the reading of a thermometer.
Mr. Eisenmann: I have observed that where the changes of temperature were sudden, a difference of 20 or 30 degrees, there was a set which was not overcome in 24 hours.
Prof. Morley: In our work our standard is first placed in an ice-box, in front of which is a refractometer. There we measure the fraction and count the whole number. When we have had it in there as long as may be required, we transfer it to our comparer, which is already in an icebox;
