to two causes: to the brushes not making contact on the commutator, which can easily be seen by pressing on the brushes with the hand, or to imperfect contact between some of the segments of the commutator and the windings, where they are soldered into it. The best way to ascertain whether this latter defect occurs is to get a dry cell and a galvanometer or voltmeter, and test the motor through from section to section of the commutator. If a point is found in which no current passes through the motor from a segment of the commutator, the conductor running to that segment ought to be properly sweated, i.e. soldered, in.
It is, perhaps, as well to explain the manner in which this kind of testing should be conducted, and the principle on which it is based; as a great number of possible faults in an electric vehicle may be detected by its means.
For an electric current to pass from the positive terminal of a battery or dry cell to the negative there must be a metallic or conducting circuit from one to the other. If the galvanometer or voltmeter be included in this circuit, then if the metallic connection be all right, on making contact to the dry cell or other battery used, the needle of the galvanometer will move. Thus, let a flexible wire be attached to one terminal of the cell, the other end of it being preferably soldered to a point mounted in a wooden handle, and let this point be pressed upon one section of the commutator. The other terminal of the cell should be connected to one terminal of the galvanometer, and the other terminal of the galvanometer to a similar metal point by a flexible wire to that first described, also preferably mounted in a handle. If this be then pressed upon the section of the commutator which should correspond with the first, then, if all be right, the current will pass from the positive of the cell through the coils of the motor, through the galvanometer, back to the negatives of the cell, and the galvanometer needle will move. This shows that there is metallic connection from one point to the other. If this connection be imperfect the needle will not move.
