Timing light connectors

I was just given a timing light and I can't work out how you use one of the connectors:



(click to enlarge)

I'm assuming this is a simple Xenon tube light. I can see the two positive and negative connectors but what's the spring for? Must be to go to the #1 spark plug but how? I've never seen one of these before. Is it just broken?

Any advice useful at this point.

 

Blue clamp on #1 plug wire. NOT the boot. Black clamp is ground. Red clamp is positive connection.

 

The spring on the blue lead goes between the plug wire and the plug on the number one cylinder. These were pre inductive timing lights.

P.S. I didn't know what it was for till I found one either

 

I understand the boot of the spark plug wire to be the end near the engine block. If it's not this end, then it must go at the distributor, am I right?
Cheers
~K

 

Yes on the block end, Just pull the wire off the plug. Small end of spring goes in the boot end of the wire and the bigger end goes on the plug.

 

The end near the block. Your answer is above you.

 

You just made all of us old guys age about twenty years! I forgot how weird some of the old tools look to the younger guys.