What name is given to the device which automatically cuts off the electricity supply during short-circuiting in household wiring?

Two coils A and B of insulated wire are kept close to each other. Coil A is connected to a galvanometer while coil B is connected to a battery through a key. What would happen if:

(i) a current is passed through coil B by plugging the key?

(ii) the current is stopped by removing the plug from the key?

A portable radio has a built-in transformer so that it can work from the mains instead of batteries. Is this a step-up or step down transformer?

What is the usual capacity of an electric fuse used (i) in the lighting circuit, and (ii) in the power circuit, of a small house?

Give the symbol of an electric fuse used in circuit diagrams.