Electrickery

[David Stepney]

Yes, there should be a light spring to keep the cutout points open. When the cutout coils are energised, the magnetic force overcomes the spring and the point close. As the dynamo ceases to charge, current starts to run from the battery back to the dynamo, which reverses the field of the current coil (the winding with the thick wire), which opposes the field in the voltage wire (the coil with the thin wire). The spring now is sufficiently strong to pull the points apart.

The spring only needs to be light. The stronger the spring, the more voltage is required to close the points.

Hope this explanation helps