Page:Popular Science Monthly Volume 90.djvu/173

 Popular Science Monthly

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��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

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