When a battery is exhausted, or nearly so, it will not magnetize the core of the coil sufficiently to induce a current that will give a strong enough spark to ignite the mixture. The length of time that a battery should last, or the mileage that it should give, is learned from experience, and when it is exhausted in less time there is evidence that it has either been short-circuited or that the current has been used too extravagantly.
The extra source of current that should be provided should not be switched into circuit until the short circuit that exhausted the regular source has been located and removed, or there will be a further waste of current. It occasionally happens that through carelessness the battery is so connected that one of the cells is reversed, in which case the current will be reduced by the loss of the current from two cells, for the energy of the reversed cell will neutralize that of another. If both sets of battery have become exhausted by short circuit, current for further running may be obtained by connecting the two sets together in multiple, supposing that each set is, as is usual, connected in series.
An apparently exhausted dry battery will recuperate, or pick up, if allowed to stand, but a storage battery that shows 1.8 volts or less for each cell should be immediately recharged, for further use will cause fatal injury.
THE MAGNETO
The principal troubles that come to magnetos are due to lack of lubrication and the weakening of the permanent magnets. The clearance between the armature and the pole pieces between which it revolves is very small, and even slight wear of the bearings through insufficient lubrication will permit the armature to touch. The oil cups with which the bearings are provided must be kept filled, or if the parts are connected to a mechanical lubricator, the proper adjustment must be maintained.
When a magnet is permitted to stand without a piece of iron connecting the poles, the magnetism will dissipate; to prevent this, a bar of soft iron, called a keeper, is placed across the poles, the magnetic lines of force flowing through it from one pole to the other, and the strength being retained. The permanent magnets forming the field of a magneto are very strongly magnetized, and the proportion of the strength lost through misuse will be large, resulting in their being weakened to such an extent that the current produced will not be sufficient for ignition purposes. These magnets can be re-magnetized by the makers when this occurs, the magneto regaining its current-producing ability.
The strength of the magnets will be retained for a much longer period if, when the car is standing, the armature is in such a position that it acts as a keeper; this will occur when its iron core is horizontal. As the armature will be in this position when the current is being produced, it is only necessary to bring the engine to a stop at the point when one of the pistons is approaching the end of a compression stroke. On leaving the car, the low speed should be engaged and the clutch withdrawn; the engine may be cranked slowly, and the armature held in position by throwing in the clutch to hold the crank shaft when compression is nearly complete.
It is most inadvisable to take a magneto to pieces without an exact knowledge of its construction. Except for lubricating the bearings, it is best to leave it entirely alone. In setting a magneto, it must be remembered that it is not delivering current continuously, and that when the iron core of the armature is parallel to the poles of the magnets no current is produced. The greatest strength of current is delivered as the core moves into a vertical position, and revolving the armature by the fingers will show this, for during part of a revolution it will turn easily, and during the rest of the revolution there will be resistance. The engine should be cranked until a piston is near the top of a compression stroke; holding it there, the magneto gear may be meshed with that on the half-time or crank shaft in such a manner that the armature core has moved a trifle past the vertical position. This will result in the production of a current at the instant that it is necessary for the ignition of the charge. (See Appendix.)
THE COIL
There is always a small spark between the vibrator contacts of a coil, and in time the platinum will be burned and corroded, the oxide that forms being an insulator and preventing the flow of the current. In such a case the points may be smoothed with a fine flat file and polished on a strip of leather, or if too far gone for this, may be renewed by soldering a short length of platinum wire to the adjusting screw and a plate of the same metal to the blade. While this job is a delicate one, it is not beyond the range of a careful amateur.