7. The diagram shows a design for an electrically operated model railway signal a) Explain, step by step, how it works.
b) Explain how, by adding a scale and a spring, you could use it as an ammeter.

(Diagram in attachment)
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diagram is missing, but i will try to explain your doubt.

answer:

amplitude is the maximum displacement or distance moved on a point of a vibrating body or wave measured from its equilibrium position.it is equal to half the length of vibration path.

current and voltage can both be different in different parts of a circuit so it matters where you place the ammeter and voltmete

float.

archimedes' principle

denser than water sinks in water.

denser than mercury would sink in it