Question

A small circular thing to activate a machine answer

Answer 1

It's condenser. I think

Answer 2

A switch is a circular thing that that is used to activate the machine.

EXPLANATION:

It is an electrical component that is used in a circuit to pass the flow of current or stop the current. The contact between the switches is one or more which may operate simultaneously or in an alternative way.

The switch also interrupts the current flowing in the machine. This switch is also called a button because sometimes it can be used as a button to operate.