Excerpt from Notes: Laboratory of Dynamo-Electric Machinery for the Use of Students in the Lowell Institute School for Industrial Foremen, Massachusetts Institute of Technology

Protection of Ammeters and Voltmeters. Ammeters are always placed in series with the circuit carrying the current to be measured. They should, therefore, be protected from excessive current due to accidental short-circuit Of the line or other cause by being kept short-circuited except when read. All ammeters used in the laboratory must be protected by short-circuit blocks provided for this purpose. One or more Of these short-circuit blocks are fastened to each laboratory table. Extra ones may be Obtained from the instrument room if needed.

Voltmeters are arranged to be placed directly across the circuit the potential of which is to be measured, and need no protection when the voltage range Of their scale is greater than the voltage Of the circuit to which they are attached. When, however, a low range scale of a voltmeter is used to measure voltage in a part Of a circuit which is supplied through resistance from a source Of higher voltage, care must be taken to prevent injury to the volt meter. If the circuit becomes accidentally opened between the points to which the voltmeter is attached, full line voltage will be put on the instrument, seriously injuring it if not actually burning it out. For this reason a voltmeter which is reading on its low volt age scale should be disconnected as soon as its reading has been recorded. A low voltage scale must never be used without first making sure that the voltage Of the part Of the circuit across which the voltmeter is to be connected is within its range. The high volt age scale should always be connected before changing to a scale Of lower range.

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