The greater part of the sulphate of copper that rises through the liquid by diffusion is carried off by the siphon before reaching the zinc, the latter being thus surrounded with an almost pure solution of sulphate of copper having a slow motion from top to bottom. This renewal of the liquid is so much the more necessary in that the saturated solution of sulphate of copper has a density of 1.166, and the sulphate of zinc one of 1.445, There would occur, then, a mixture through inversion of densities if the solution were allowed to reach a too great amount of saturation, did not the siphon prevent such a phenomenon by sucking up the liquid into the part where the mixture tends to take place. The chemical action that produces the current is identical with that of the Daniell element. In its application, this pile is considerably modified in form and arrangement. Each element (Fig 2) consists of a flat wooden hopper-shaped trough, about fifty centimeters square, lined with sheet lead to make it impervious. The bottom is covered with a sheet of copper and above this there is a zinc grate formed of closely set bars that allow the liquid to circulate. This grate is provided with a rim which serves to support a second similar element, and the latter a third, and soon until there are ten of the elements superposed to form series mounted for tension. The weight of the elements is sufficient to secure a proper contact between the zinc and copper of the elements placed beneath them, such contact being established by means of a band of copper cut out of the sheet itself, and bent over the trough. [Illustration: FIG. 2.--THE THOMSON PILE. (Siphon Recorder Type.)] On account of the large dimensions of the elements, and the proximity of the two metals, a pile is obtained whose internal resistance is very feeble, it being always less than a tenth of an ohm when the pile is in a good state, and the electromotive force being that of the Daniell element--about 1 08 volts. Sometimes the zinc is covered with a sheet of parchment which more thoroughly prevents a mixture of the liquids and a deposit of copper on the zinc. But such a precaution is not indispensable, if care be taken to keep up the pile by taking out some of the solution of sulphate of zinc every day, and adding sulphate of copper in crystals. If the pile is to remain idle for some time, it is better to put it on a short circuit in order to use up all the sulphate of copper, the disappearance of which will be ascertained by the loss of blue color in the liquid. In current service, on the contrary, a disappearance of the blue color will indicate an insufficiency of the sulphate, and will be followed by a considerable reduction in the effects produced by the pile. The great power of this pile, and its constancy, when it is properly kept up, constitute features that are indispensable for the proper working of the siphon recorder--the application for which it was more especially designed.