EXPERIMENTS WITH THE DISPLACEMENT INTERFEROMETER. 19 



pendulum exceeded the temperature effect, and because the necessary stirring 

 of the water in the float interfered with the free play of the pendulum. The 

 temperature effects obtained were as liable to be positive as negative. In 

 fact, it is conceivable that although direct effect of temperature may not be 

 serious, the indirect effect produced by the friction of -irregular convection 

 currents of water on the float may be so. Unfortunately no means of allowing 

 for these suggests itself, so that constancy of temperature is a condition for 

 the proper functioning of the floated pendulum. Symmetrical occurrences 

 would of course be ineffective. 



It will now be advisable to resume the equation of moments in 7 , where 

 the torque is fully expressed as 



if the weight mg is put on the pan of the float at a distance / from the vertical 

 axis, T'T = mgl<f>. Consequently if p does not vary, the effect of e vanishes, 

 whence 



ml 



n 



M(e'-ey 



the distance of the center of gravity from the axis, may be found. The meas- 

 urements below show this to be a good method under proper precautions. 



A number of experiments of this kind were made with the pendulum mod- 

 ified by the addition of a damper on the left, which would throw the center of 

 gravity slightly in that direction. The first two series of observations com- 

 pared the deflection x with the water damper attached and the float either 

 fully or less than half submerged, respectively. The variable data obtained 

 for x and their small value showed that capillary forces were in play which 

 completely vitiated the use of the pendulum. The zero was not steady. 

 There seemed to be an actual capillary resistance in play. Hence the h 

 obtained was too large. The damper at the end of the arm, notwithstanding 

 its convenience, is therefore not admissible as an attachment under the 

 conditions of sensitiveness of the pendulum. 



With the damper removed, the float being but half submerged, the results 

 were improved, but the capillary forces on the float were still excessive. It was 

 not until the float was completely submerged that the capillary forces were 

 negligible and consistent values of h were obtained, the accuracy of which 

 might easily have been improved by closer observation. 



The addition of over 500 grams weight of buoyancy has no effect on the 

 deflection, if the error due to capillary forces is allowed for. 



If the torque applied is constant while the temperature of the water in which 

 the float is submerged changes, the differential equation becomes dd/dp = 

 tV/M or 



dx = s^D ( V/M)dp 



