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Rh an inch, which is approximately the thickness of a thumb nail. An English firm has recently introduced a plug which contains no breakable insulators; this is an undoubted improvement, provided the insulating material employed is found to stand the strain well.

Dirty Commutator.—Clean all contacts from oil and dirt. Most commutators are so placed as to give the maximum possible opportunity to collect oil and dirt. They should always be provided with a cover.

Batteries.—In course of time the batteries will become weak or discharged. Always carry a spare set. A two-way



switch should be provided on the car so that in a moment the spare set can be brought into use. The diagram shows the method of connecting up the swatch, while both batteries may be charged together without interfering with the connections.

(2) .—From the list of possible failures given in the last section, it will be inferred that there are many faults liable to occur owing to the multiplicity of wires, batteries, coils, and the like. To obviate these difficulties, electricians have designed a little machine known as the 'magneto-generator.'