Ans. If the machine be working on the same circuit with other machines, or with a storage battery, it is, or course, necessary to make the voltage of the machine equal to that on the line before connecting it in the circuit. If the machine work alone, the switch may be closed either before or after the voltage comes up. The load will be thrown on suddenly if the switch be closed after the machine has built up its voltage, thus causing a strain on the belt, and possibly drawing water over the engine cylinder. On the other hand, if the switch be closed before the voltage of the machine has come up, the load is picked up gradually, but the machine may be slow or may even refuse to pick up at all. Ques. Why does a shunt machine pick up more slowly if the main switch be closed first? Ans. Because the resistance of the main line is so much less than that of the field that the small initial voltage due to the residual magnetism causes a much larger current in the armature than in the shunt field. If this be too large, the cross and back magnetizing force of the armature weakens the field more than the initial field current strengthens it, and so the machine cannot build up. Ques. If a shunt dynamo will not pick up, what is likely to be the trouble? Ans. The speed may be too slow; the resistance of the external circuit may be too small; the brushes may not be in proper position; some of the electrical connections in the dynamo may be loose, broken or improperly made; the field may have lost its residual magnetism. [Illustration: Figs. 676 and 677.--Diagrams of ground detectors. Fig. 676, a ground detector switch suitable for mounting on a switch board. The two arms pivoted at their upper ends are connected with an insulating bar A and make contact at their lower ends with two brass strips and a contact button, which are connected to the bus bars and ground, respectively. When the arms are moved to the left, the positive bus bar is connected to the ground through the voltmeter V. In fig. 677 is another form of ground detector. This is known as a lamp ground detector. On a 110 volt system two ordinary lamps are connected in series, while the line connecting the lamps is connected to the ground through a snap switch S. When current is on, the two lamps will burn with equal brilliancy, but at a lower candle power. When the switch S is closed, if the two lines be clear, the brilliancy of the lamps will not be affected, but if there be a ground on the positive side, one lamp will burn brighter, the brightness depending on the resistance of the ground. If there be a dead ground, the lamp will burn to its full candle power.] Ques. What is the indication that the connections between the field coils and armature are reversed? Ans. If the machine build up when brought to full speed, the connections are correct, but if it fail to build up, the field coils may be improperly connected.