H Exp – Electricity and Electronics 07 – The out-of-balance Wheatstone bridge – applications

Aim – To investigate several applications of the Wheatstone bridge in the out-of-balance condition.

Instructions

1. The Wheatstone bridge is balanced.

▪ Place the LDR in a dark place and note the current on the micro-ammeter.
▪ Place the LDR in a light place and note the current on the micro-ammeter.
▪ Explain why the current is greater than zero for one condition and less than zero for the other.
▪ Replace the LDR with the thermistor.

2. The Wheatstone bridge is balanced when the thermistor is in melting ice.

▪ Place the probe in the boiling water by clicking on the text “Boiling water at 100 ℃”.
▪ Note the current on the micro-ammeter.
▪ Predict the current that will be obtained when the probe is at room temperature.
▪ Test your prediction. What assumptions are you making about the temperature of the probe, its resistance, and the out-of-balance current?
▪ Replace the thermistor with two strain gauges connected in the two arms of the Wheatstone bridge as shown.
▪ Click above the right end of the hacksaw blade to push it down. Click below to push it up.
▪ Note the out-of balance current.
▪ Explain how the out-of-balance current is used to show:
▪ a) the amount of bending / strain put on the hacksaw blade
▪ b) the direction of the bending / strain put on the hacksaw blade.

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