110 THE INTERFEROMETRY OF 



ture of about o.i degree between the tubes should be easily observable by 

 mere displacement, whereas a difference of less than 0.03 would be equivalent 

 to the passage of one interference ring. 



Again, from equation (2), if A/V = icr 4 cm., then A// = io~Yo.35i = 10^X2.8 

 cm., or about gX icr 5 cm. per vanishing interference ring are the displacements 

 to be anticipated. These are equivalent to pressures of about 0.3 and o.i 

 dyne per square centimeter. 



59. Observations. A large number of observations were made with the 

 apparatus described, but as under present surroundings the fringes always 

 quivered violently, no quantitative results of value were obtained. Naturally 

 such experiments imperatively demand a laboratory remote from traffic, since 

 the undulation of mobile liquid surfaces is introduced in addition to the tremors 

 of solid appurtenances. 



An attempt was made to register the pressure near an electrically charged 

 point, but no results could be obtained. Again, though the attraction of an 

 electrically charged surface for the free surface of water in either tube was 

 recognized, on using adequately high potentials to measure the forces the 

 surface became troubled and the fringes vanished. In this case, if p is the 

 pressure, V the difference of potential in volts, and d the distance apart of 

 surfaces in centimeters, /=4-4 Xicr 7 X( V/d) 2 dynes/cm. 2 if 7=0.3 can just be 

 determined by the displacement method, and if V = 8o volts (roughly) is the 

 smallest potential difference discernible for quiet fringes. Finally, Dulong 

 and Petit 's experiment gave very definite results even for small ranges of 

 temperature, subject to the conditions stated. 



By surrounding the top of the tube C, figure 71, with a close-fitting helix, 

 the upper face of which reached just below r, while the surface of liquid within, 

 w, lay at its center, an attempt was made to detect the susceptibility k of 

 water, etc. If the field of the coil be written roughly H = o.^irin/l, where i 

 is the current in amperes, n the number of turns, and / the length of the helix, 

 we may write 



Since p = i , the increment of head, h, becomes 



A/t = kH*/g = ( 



Hence if roughly k= io~ 6 , g= io 3 , i=i am., w// = 3S as in the helix used, 



cr 6 cm. 



Thus if 2 = 10 amperes, A/i = 2Xio^ cm., nearly, and easily determinate by 

 the displacement of ellipses, or from interference rings. The experiment was 

 tried, but the quiver of rings was such as to admit of no decision. In case of 

 the magnetic solutions, k is of course much larger; but under the circumstances 

 it did not seem worth while to attempt further work. This will be done with 

 other apparatus in the course of this paper. 



