Uses of meters Galvanometers are used to measure small currents, in the range of microamperes to milliamperes. Ammeters measure currents greater than those measured by a galvanometer. Voltmeters measure the potential difference across a circuit. Ohmmeters measure the resistance across any resistor or electronic device.
EXERCISE 24.1 CONVERTING A GALVANOMETER Æ
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Q1 A galvanometer has a resistance of 25 and gives a full-scale deflection with a current of 10 mA. Find the resistance of the shunt that must be connected in parallel to enable the galvanometer to measure currents of up to 6A.
Q2 A moving-coil galvanometer has a resistance of 2 and a full-scale deflection of 6 mA. Calculate the size of the resistor required to convert it into an ammeter with a full-scale deflection of 1 A.What is the effective resistance of the ammeter?
Q3 Find the value of the resistor that must be connected in series with a galvanometer to convert it to a 6 V max voltmeter, given that the galvanometer has a resistance of 6 and gives a full-scale deflection with a current of 20 mA.
Electromagnetic induction is where an emf is induced by a change in the magnetic flux linking a circuit.
Adjustable spark gap Soft iron core
Platinum contacts
Primary coil
Secondary coil
Pivot Fig 24.11: Induction coil
Lenz’s law and Faraday’s law combine to give: , E is the induced emf, is the magnetic flux, t is the time,
E Ndf dt
f
where N is the number of turns in the solenoid. The ‘–’ signifies that the induced emf ‘opposes’ the change causing it.
Induction Coil INDUCTION COIL
An induction coil is an electronic device used to get a very high voltage from a low voltage d.c. source.
An induction coil is made up of: • A d.c. power supply of relatively low voltage • A primary coil of thick wire, so as to have low resistance, allowing a large current to flow and also with very few turns compared with a secondary coil
