Bit of a word problem here - usually a device is named a "solenoid" when it is necessary to convert electrical
energy into a physical linear motion such as operating a "plunger" in a starter solenoid that physically moves the Bendix gear
and also engages large copper contacts that allow very large amperages to flow to the starter motor
- - "Relays" are similar devices but the plunger only closes a very light weight set of contacts that allow a large electrical current
to be controlled by a very small electrical current
. That is, the starter switch uses very little electrical power to close the relay - which enables the use of small electrical wiring
. Then larger electrical wiring
from the relay powers the solenoid which physically engages the starter motor
to the flywheel and closes large contacts to allow huge amounts of power from the battery to rotate the starter motor.
- - Modern cars and boats make use of "solid state" relays and thin wiring to power larger relays downstream close to the actual high power load. This all saves significant money
and adds additional layers of safety
to the wiring system.