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CH. XIII] ELECTRIC MEASUREMENTS 481
the dynamo, any point in the circuit toward or nearer this positive pole will be positive in comparison with any other point nearer the negative pole. Then if the circuit is cut at any point the end of the wire next the positive pole of the dynamo will be positive, and the end nearer the negative pole of the dynamo will be negative. Now if the cut end of the wire nearer the positive pole of the dynamo is inserted in the negative binding post of the ammeter, and the other end in the other binding post, the needle tends to be deflected in the wrong direction. If the two ends of the wire are correctly connected with the ammeter, the needle will be deflected in the right direction, and indicate the amperage.
§ 666. Ammeter for projection. — In projection the ammeter is usually all that is required, for the voltage on a given line is nearly constant, and can be found easily by inquiring of the central station. On the other hand, the required amount of current for different purposes varies greatly and the factors in the production of a good image are so many that an ammeter to show at a glance what amount of current is flowing is of the highest importance, for with a given amount of current the operator knows at once what kind of a light can be reasonably expected in the different cases. If the screen light is not good with the adequate amperage for the purpose then he can look to the other possible causes of failure (see § 61-96).
If one is to be able to determine for himself all the electric factors in projection work, then a voltmeter and a wattmeter should be added to his apparatus.
§ 667. Precautions for the ammeter. — In connecting the ammeter be sure not to connect the ammeter directly to both line wires. As the ammeter has very little resistance, putting it across the line would have practically the same effect as connecting the two points with a heavy wire, that is a short circuit would be formed and the fuses would be blown. Besides the very heavy current which would flow momentarily might be sufficient to seriously damage the delicate instrument.
§ 668. Safe rules for the beginner to follow when connecting instruments may be stated as follows: