Above is the position of the cam at either 29 or 59 minutes past the hour.
Both contacts are being pushed upwards by the cam which is now at its highest point. The two contacts are not touching each other though and the mercury switch's electro magnet is not yet energized.






Above is the position at either 29 minutes & 30 seconds or 59 minutes & 30 seconds past the hour.
The gravity arm has just been reset and the pilot dial has stepped on 30 seconds, causing the cam to rotate one step in the direction of the arrow. The top contact (red) has now dropped of the edge of the cam and is now resting against the blue one underneath. This is because the blue one is still being pushed upwards as it is still on top of the cam. The circuit is now made, and the electro-magnet for the mercury switch is energized, pulling the mercury tube to the 'almost on' position.

(For an explanation of the sequence of events I am referring to, go to the Synchronome Power Station Operation page)



At some stage in the next 30 seconds (dependant on the length of pulse that the movement has been set up for) the additional vane, that these models are equipped with, will tip the mercury switch to its fully 'ON' position.




Above is the position at either the half hour or on the hour.
Once again the gravity arm has reset and the pilot dial has stepped on another 30 seconds, moving the cam onto its next position. This has caused the blue contact to slip of the edge of the cam. Consequently because it is no longer being pushed upwards, it has parted company from the red contact and the electro-magnet circuit is broken. This allows the mercury tube to return to its normal position.





Here you can see the actual contacts themselves, they do not align vertically like most contact sets do. Their action is that the small rod on the top contact comes down to rest on the contact pad of the lower spring.