It was concluded by Mr. Reid from the above experiments that the wax applied to the rim of the jar is totally ineffective and has no merits.  The wax seal loosens around the edges and does not totally prevent creeping of the zinc sulphate salts, although nearly so.  The wax-sealed jar must have holes drilled in it to allow the gases to escape.  The method is hardly commercial, as it is difficult to make a neat appearing cell, besides making it almost impossible to manipulate its contents.  A coat of paraffin oil approximately 1/2 inch in thickness (about 6 ounces) gives perfect protection against evaporation and creeping of the zinc sulphate salts.  The cell, having the paraffin-oil seal, had a very neat, clean appearance as compared with cells No. 1 and No. 2.  It was found that the zinc could be drawn out through the oil, cleaned, and replaced with no appreciable effect on voltage or current.

Setting Up:—­In setting up the battery the copper electrode is first unfolded to form a cross and placed in the bottom of the jar.  Enough copper sulphate, or blue-stone crystals, is then dropped into the jar to almost cover the copper.  The zinc crowfoot is then hung in place, occupying a position about 4 inches above the top of the copper.  Clear water is then poured in sufficient to fill the jar within about an inch of the top.

If it is not required to use the cell at once, it may be placed on short circuit for a time and allowed to form its own zinc sulphate.  The cell may, however, be made immediately available for use by drawing about one-half pint of a solution of zinc sulphate from a cell already in use and pouring it into the jar, or, when this is not convenient, by putting into the liquid four or five ounces of pulverized sulphate of zinc, or by adding about ten drops of sulphuric acid.  When the cell is in proper working condition, one-half inch in thickness of heavy paraffin oil of good quality may be added.

If the blue line gets too low, and if there is in the bottom of the cell a sufficient quantity of sulphate of copper, it may be raised by drawing off a portion of the zinc sulphate with a battery syringe and replacing this with water.  If the blue line gets too high, it may be lowered by short-circuiting the cell for a time, or by the addition of more sulphate of zinc solution from another battery.  If the copper sulphate becomes exhausted, it should be replenished by dropping in more crystals.

Care should be taken in cold weather to maintain the temperature of the battery above 65 deg. or 70 deg.  Fahrenheit.  If below this temperature, the internal resistance of a cell increases very rapidly, so much so that even at 50 deg.  Fahrenheit the action becomes very much impaired.  This follows from the facts that the resistance of a liquid decreases as its temperature rises, and that chemical action is much slower at lower temperatures.

The gravity cell has a practically constant voltage of 1.08 volts.  Its internal resistance is comparatively high, seldom falling below 1 ohm and often rising to 6 ohms.  At best, therefore, it is only capable of producing about 1 ampere.  The gravity cell is perhaps the most common type of cell wherein depolarization is affected by electro-chemical means.