Popular Science Monthly 



157 



a water-particle well within the drop would 

 be attracted from all sides, and hence the 

 fluidity of the drop would not be affected. 

 But the surface molecules would have no 

 balanced attractions because the body of 

 the water-drop is within the surface. 

 Consequently the surface particles are 

 pulled inward toward the center of the 

 water, producing a tension at the film of 

 water at the surface so that it is capable 

 of supporting a certain amount of liquid. 



What the Danish scientist has dis- 

 covered is that even a very slight electric 

 current, passing through the globular 

 supporting-surface or film of a liquid, is 

 sufficient to affect its cohesion. The 

 natural "surface-tension" — as it is called — 

 is strained, and in the case of mercury the 

 globulizing effect is diminished and the mer- 

 cury becomes more capable of adhering to 

 and netting the surfaces which it contacts. 

 In the form shown in Figure i, the 

 mercury-jet is brought into contact with 

 a bone-surface B which is moistened 

 slightly with a solution of caustic potash. 

 By passing the very feeble electric currents 

 through the wetted bone and thereby 

 through the mercury-jet, the jet is enabled 

 to "wet" the bone-surface to a greater 

 degree than before. Consequently, it is 

 slightly deflected and thereby, by means of 

 a lever arrangement L, a very strong 

 amplification of signals can be produced by 

 the pen P on the recording-tape T. 



In another form, shown in Figure 2, a 

 bone-collar C is provided, which is made to 

 surround the mercur>'-jet and to come in 

 contact with it. As is well known, every 

 jet sooner or later breaks up into separate 

 globules because the surface envelope of 

 the mercury-column is able to support only 

 a definite amount of liquid. 



The contact arm A in the circuit con- 

 taining the relay R is raised upward just 

 until the mercury-column is continuous 

 between the collar C and the end of the 

 arm A. Then with the passing of the 

 signaling current through the collar and 

 the mercury-column, the cohesion between 

 the particles of mercury being reduced, it 

 is impossible for the jet to hold together 

 as well as before, and the consequence is 

 that the globulization occurs for a smaller 

 column length than before. The circuit 

 containing the relay R is interrupted by 

 the spaces between the globules. The re- 

 lay R is so connected to another circuit 

 containing the recording-apparatus (not 

 shown) that a powerful current is produced. 



Constructing a Simple Variable 

 Condenser Switch 



THE details for a simple variable con- 

 denser switching mechanism is illus- 

 trated, which, if used with a properly made 

 condenser, will produce good results. A 

 true zero capacity can be obtained with 

 this type of mechanism. It may be used 

 as a separate instrument on a condenser, 



Variable switch mechanism for controlling 

 output of a condenser for sending or receiving 



or it may be mounted as a part of a cabinet 

 set. In building this apparatus dimensions 

 may be used to suit the needs, as any con- 

 denser, either sending or receiving, may be 

 connected on this principle. One size of 

 this form of mechanism is given as aH 

 example. The materials necessary are as 

 follows: 



2 strips of wood, 5 in. long, ^ in. wide and J^ 



in. thick 

 2 strips of wood, 5 in. long, }4 in. square 

 2}-^ ft. of spring-brass wire, No. 24 gage 

 I piece of j^-in. brass rod 6 in. long 

 I insulating knob to fit brass rod 

 I cover for condenser, about 5 in. long 

 Several finishing nails % in. long 



The four long strips of wood are nailed 

 in the form of a square tube, as shown in 

 Fig. I. If the condenser is to have 10 

 different capacities, mark the tube into 10 

 divisions, starting the first mark about 2 

 in. from the end and dividing the other 

 spaces equally. With this part completed 

 take a hack or scroll-saw and cut into each 

 of the divisions diagonally, so that the cut 



