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 fac- 

 tors in projection work, then a voltmeter and a wattmeter should be 

 added to his apparatus. 



667. Precautions for the ammeter. In connecting the am- 

 meter 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: 



