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National Projector Carbons
wattmeters are necessarily of somewhat delicate construction and the best instrument may be ruined by
misuse.
Meters should never be placed where they will be exposed to high temperature, vibration, or strong magnetic fields. Such conditions may quickly impair their accuracy.
They should be calibrated at regular intervals, at least once a year.
The projectionist should not attempt to adjust or repair electrical meters. When attention is required they should be returned to the manufacturer who has proper facilities for making the needed repairs and adjustment.
Make sure that the contacts on all external connections to electrical meters are clean and firmly secured.
Damage to rheostats sometimes results from operation with a short arc at an arc voltage appreciably below normal. This practice throws on the rheostat a greater proportion of the line voltage than it is designed to carry, so that the arc current can be held down to normal value only by cutting out some of the parallel connected resistors. This may force the remaining resistors to absorb so much wattage in excess of their radiating capacity that destructive temperatures result. With correct arc adjustment, ample range of current control should be afforded by normal operation of the rheostat.
Careful attention to the foregoing precautions, and to the more specific instructions given in earlier chapters for the type of lamp in use, will afford the projectionist using National Projector Carbons screen illumination of maximum steadiness, brilliancy and uniformity.
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