Numerous voltaic combinations and varieties of cell have been found out. In general, where-ever two metals in contact are placed in a liquid which acts with more chemical energy on one than on the other, as sulphuric acid does on zinc in preference to copper, there is a development of electricity. Readers may have seen how an iron fence post corrodes at its junction with the lead that fixes it in the stone. This decay is owing to the wet forming a voltaic couple with the two dissimilar metals and rusting the iron. In the following list of materials, when any two in contact are plunged in dilute acid, that which is higher in the order becomes the positive plate or negative pole to that which is lower:-- POSITIVE Iron Silver Zinc Nickel Gold Cadmium Bismuth Platinum Tin Antimony Graphite Lead Copper NEGATIVE There being no chemical union between the hydrogen and copper in the zinc and copper couple, that gas accumulates on the surface of the copper plate, or is liberated in bubbles. Now, hydrogen is positive compared with copper, hence they tend to oppose each other in the combination. The hydrogen diminishes the value of the copper, the current grows weaker, and the cell is said to "polarise." It follows that a simple water cell is not a good arrangement for the supply of a steady current. The Daniell cell is one of the best, and gives a very constant current. In this battery the copper plate is surrounded by a solution of sulphate of copper (Cu SO4), which the hydrogen decomposes, forming sulphuric acid (H2SO4), thus taking itself out of the way, and leaving pure copper (Cu) to be deposited as a fresh surface on the copper plate. A further improvement is made in the cell by surrounding the zinc plate with a solution of sulphate of zinc (Zn SO4), which is a good conductor. Now, when the oxide of zinc is formed by the oxygen uniting with the zinc, the free sulphuric acid combines with it, forming more sulphate of zinc, and maintaining the CONDUCTIVITY of the cell. It is only necessary to keep up the supply of zinc, water, and sulphate of copper to procure a steady current of electricity. The Daniell cell is constructed in various ways. In the earlier models the two plates with their solutions were separated by a porous jar or partition, which allowed the solutions to meet without mixing, and the current to pass. Sawdust moistened with the solutions is sometimes used for this porous separator, for instance, on board ships for laying submarine cables, where the rolling of the waves would blend the liquids.