In making connections between cells insulated wire should be used, or special battery connectors are preferably employed. The ends of the wires or connectors and the binding posts must be scraped clean so that good electrical connection can be made between the two, and the knurled nuts should be screwed tight into place. Care must also be taken that the pasteboard covering around the battery is not torn. This would allow contact between the zinc containers, and thus short-circuit the cells. The batteries should be placed so that the zinc cans and the binding posts of any cell do not come into contact with any other cell. Vibration might cause enough motion for the brass terminal to wear through the pasteboard of the neighboring cell and make contact with the zinc can. Different classes of work require different amounts of current at different voltages and by choosing the proper combination of series, multiple, or series-multiple connections practically every requirement can be fulfilled. For electric bells, telegraph instruments, miniature lights, toy motors with fine wire windings, etc., series connection is recommended for the reason that the resistance of the external circuit is high and a large voltage is necessary. For spark coils, magnets and toy motors with large wire windings, multiple or series-multiple connection of batteries should be used as a high voltage is not required. For some work, gas-engine ignition especially, it is economical to have two complete sets of batteries, either of which can be thrown into the circuit at will, so that while one set is delivering current the other is recuperating. It has been estimated that by using two sets of batteries, properly connected to give the desired current, the life of each set is increased about four times. Thus it is seen that a saving of 50 per cent. is effected in the cost of the batteries. =Battery Testers.=--The “strength” of a cell is determined by the amperes of current it is capable of producing; therefore, a meter that will indicate the amount of current being produced is used to test the current strength of the cell. Battery testers are made to indicate voltage or amperage and sometimes the instrument is made to indicate both volts and amperes. As explained above, the voltage of a cell is not a true indicator of its strength. The ampere meter or ammeter, as it is termed, is the proper indicator of the strength of the cell. [Illustration: FIG. 257.--Battery tester.] The common battery tester does not always give the exact number of amperes of current, but it indicates the relative strength which is really the thing desired. When the current from an active cell is once shown on the dial of the tester, any other cell of the same intensity will be indicated in like amount.