Construction of Wright Maximum Demand Indicator

Construction of Wright Maximum Demand Indicator Points : Construction of Wright Maximum Demand Indicator Such an instrument consists of a glass U-tube having two bulbs of similar capacity at their upper ends as shown. The glass bulb on the left side of the tube consists of a metallic strip around it, which being connected in series with the load circuit, carries the load current, thus it forms a heater and heats the air inside this bulb. Just below the bulb on right side of the tube, there is a side tube leading to a narrow bore index tube fitted with a scale along it. The glass U-tube contains a liquid of low temperature co-efficient of expansion; dilute sulphuric acid as an indicator. The constructions and traps in the limbs of the tube are to prevent the passage of air from one bulb to the other.

When the indicator is set initially, the index tube contains liquid up to zero mark and the right hand tube contains the indicating solution up to the level of index tube. Now when the current flows through the heater, the air inside the bulb consisting of the heater expands and causes the liquid of right hand limb to spill into index tube.

The greater the magnitude of current passing through the heating element of the indictor, the greater the expansion of liquid, and therefore, grater is the quantity of liquid overflowing in to the index tube. Now there will be not further overflow of the liquid in to the index tube until and unless the current passing through the heating element of the indicator surpasses the previous value. Hence the liquid in the indeed tube gives a true record of the maximum current drawn by the consumer during any given period.

Instrument does not record the momentary heavy current, since the instrument operates on the principle of heat production, which takes time.