8. Dry cells should be tested with an ammeter, care being taken to do it quickly as the ammeter being of a very low resistance short circuits the cell. A volt meter is not used in testing because, while the cells are not giving out current, their voltage remains practically the same, and a cell that is very weak will show nearly full voltage. When no ammeter is at hand, the battery current may be tested by disconnecting the end of one of the terminal wires and snapping it across the binding post of the other terminal; the intensity of the spark produced will indicate the condition of the battery. =Points Relating to the Care of Cells.=--To get the best results from primary cells, they should receive proper attention and be maintained in good condition. The instructions here given should be carefully followed. [Illustration: FIGS. 64 to 66: Various carbons; fig. 61 Cylindrical form; fig. 65 Calland star; fig. 66, wheel.] =Cleanliness.=--In the care of batteries, cleanliness is essential in order to secure best results. Zincs and coppers should be thoroughly cleaned every time a cell is taken out of use. The zinc, after being thoroughly cleaned, should be rubbed with a little mercury. This prevents local action. Porous cups should be soaked in clean water four or five hours and then wiped dry. The terminals of each cell should be thoroughly cleansed and scraped bright so as to get good contact of the connecting wires and thus avoid extra resistance in the circuit. =Separating the Elements.=--Obviously the positive and negative elements of a cell must not be in contact within the exciting fluid; they should be separated by a space of 3/8 to 1/2 inch. In the case of cells without porous cups, periodic attention must be given to ensure this condition being maintained. [Illustration: FIGS. 67 to 69.--Various zincs: fig. 67 Crowfoot; fig. 68 Lockwood; fig. 69 fire alarm.] =Creeping.=--As evaporation of the electrolyte takes place in a cell, it increases in strength, and crystals are left on the sides of the jar previously wetted by the solution, the action being very marked when the solution is a saturated one. The space between these crystals and the side of the jar acts as a number of capillary tubes, and draws up more liquid, which itself evaporates and deposits crystals above the former ones. So that finally the film of crystals passes over the edge of the jar and forms on the outside, thus making a kind of syphon which draws off the liquid. This action may, to a great extent, be prevented by warming the edges of the glass, or stoneware, jars, and of the porous pots, before the cells are made up, and dipping them while warm into some paraffin wax melted in warm oil, a precaution that should always be carried out when a dense solution of zinc sulphate is employed in the cell.