12 



DISPLACEMENT INTERFEROMETRY APPLIED TO 



In the case of an experimental apparatus improvised for the purpose, and 

 without a guard-ring, and in which the rigorous parallelism between electrodes 

 could not be guaranteed, a closer approach was not to be looked for. The 

 equation may be taken as trustworthy within its range. 



12. Improvements and miscellaneous experiments. The electrometer 

 eventually took the form shown in figure 13, which gives the apparatus in 

 connection with the electrophorus and a commutating key similar to Mas- 

 cart's. To put the mercury M to earth, a steel screw S, which also carries a 

 flat clamp for fastening one end of the earthed wire, has been inserted. This 

 screw 5 has the further important purpose of damping the oscillations of the 

 mercury M or M' t by adjustably closing the channel m. The deflections can 

 thus be made quite dead-beat, which is an advantage. To level the electrodes 

 C, C 1 (using a small spirit-level placed on them), each has a connecting-rod d, 

 which carries a clamp at one end, allowing the rod aa' to slide up and down, 

 rotate around a, a' and d, d f , and admitting of small displacements along d. At 

 the other end of each rod is a flat vertical plate, which is received in a fissure at 

 the top of the corresponding hard-rubber post k, k' and clamped. This gives a 

 horizontal axis at right angles to the preceding. The lower ends of the posts 

 k, k' are suitably clamped to the tubes t, t f , attached to the body B of the 

 electrometer. Here further motion along the tubes t, t' and rotation around 

 them is possible. In this way it is not difficult to place C, C' symmetrically 

 above the mercury pools (which must be shallow) and parallel to their upper 

 faces, for experimental purposes. It is not sufficient, however, if precision is 

 required, nor well adapted for measuring the distance between glass face and 

 electrode. In such a case micrometers at the inner ends of d, d' would be 

 needed with a more efficient method of leveling. I made the endeavor to 

 change the position of surfaces of the mercury, by putting M, M' in connection 

 with a third vessel on the outside, which could be raised or lowered by a micro- 

 meter-screw; but the method was not satisfactory in the long run, as the 

 apparatus shifted its zero-point and sensitiveness. To control the latter in 

 this way proved to be inadvisable and was eventually abandoned. 



Figure 13 shows the Mascart key below on the right, which consists of the 

 elastic brass strips /, I', the earthed cross-bar n above them, and the cross- 

 bar q below. All metal parts are carried 

 on high, insulating hard-rubber posts 

 / and /' at their right ends and they are 

 both in contact with n if not depressed 

 by the insulating knobs at their left 

 ends. The figure shows that / and /' 

 are electrically connected with C and 

 C' by insulated wires, while 5 is con- 

 nected with n. Thus the whole U-tube is earthed when not in use. 



The bar q is connected by wires with the brush A of the electrophorus shown 

 on the left, so that when / or /' are depressed into contact with q, C or C' 



