46 DISPLACEMENT INTERFEROMETRY BY 



The results are given in table 4 and figure 46. The constant of the dyna- 

 mometer was now C lo^Xo.Sy relative to amperes and the total resistance 

 in circuit 710 ohms. The frequency was the same as before. The same 

 bifilar wires (phosphor bronze, 0.025 cm - m diameter and 50 cm. long) sus- 

 tained the vibrations. 



The results are a considerable improvement on the preceding and the dis- 

 crepancies as a rule lie within sXio- 6 ampere. They are much more liable 

 to be in the dynamometer than in the vibrator, as the former was not well 

 adapted for these small currents. Curiously enough, the deflections begin 

 with 3 scale-parts and not at zero. As the slit-image was about i scale-part 

 broad, it is difficult to assign a reason for this. Like the slit-breadth, however, 

 it appears merely as an initial constant and is thus not of much consequence. 



If we compute the coefficient of induction as 



A being a differential symbol) from the first and fifth, second and sixth, etc., 

 observations, the results for Lu are given in the corresponding column. L 

 increases as if the resistances 2,000 to 30,000 ohms were not induction-free, 

 implying a larger L at the higher resistances; but as the current is not har- 

 monic, the reason may have to be sought elsewhere. After 10,000 ohms (or 

 even below) the induction effect is practically negligible in comparison with the 

 resistance, as appears from the column for (R-\-r)i = E. Hence the effective 

 E is constant and about n volts, implying a maximum voltage of 15 volts. 



30. Effect of frequency. A special mercury interrupter was now made as 

 shown in figure 47, having as its distinctive feature contrivances by which the 

 mercury surface could always be kept clean and bright and furthermore 

 adapted to give different frequencies. This consisted of a block of wood A, 

 figure 47, impregnated with resinous cement, with a vertical hole to receive 

 the mercury m, distilled water w, and the vibrator, eas. The spring 5 was 

 actuated as usual by an electro-magnet (not shown) and the terminals e, /, 

 for passing current through the mercury were of platinum, e being adjustable 

 at the clamp a. In view of the glass stopcock b and the pipe c d, the water w 

 could be withdrawn whenever desirable and the mercury surface washed by 

 aid of a small wash-bottle. The apparatus functioned admirably for days, 

 frequent washing presupposed. Different frequencies were obtained by sliding 

 a weight along ss. These were estimated from the moments of inertia as n = i o, 

 15, and 20. The latter could just be counted in groups of 4 vibrations with a 

 stop-watch. Higher frequencies were obtainable by using a stiffer spring 55. 



The results obtained with this apparatus are shown in figures 48, 49, and 

 50, for the phosphor-bronze bifilar differently stretched. All give evidence 

 (fig. 51) of the peculiar fact that the sensitiveness increases in marked degree 

 with the frequency. In figures 48 and 49 the telephone used was the original 

 one consisting of a file-blade and appropriate bobbin. The sensitiveness for 

 the three frequencies is respectively in ocular scale-parts 5o per milliampere. 



