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four arms of the Bridge have about 800 ohms resistance ; the three arms in the instrument case are wound with " eureka” or constantan having practically no temperature coefficient, and the fourth arm or thermometer coil of nickel or copper. The scale is marked in degrees Centigrade or Fahrenheit, and for cold storage work ranges from about 10° Fahr. ( —15° C.) to say 100° Fahr. (40° C.) ; these ranges, however, can be considerably modified. A continuous current at about 30 volts is required to work the apparatus, this can be obtained either from a small hand generator, or from the mains by use of a resistance.

Circuit Breakers (Fig. 20).—Of various types; maximum, minimum, and reverse current, and various combinations maximum and reverse current and maximum and no volt.

The loose or free handle type is fitted with laminated contacts working at a low current density. The circuit cannot be closed if the short or overload is still on. The breaks are in all cases made on carbon blocks, which are easily replaceable. A time-limit device, in which the overload produces a release of the circuit breaker in a time inversely proportional to the amount of over-load, and a magnetic blow out can be provided.

FIG. 20.

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