Old smith rev counter, where does this wire go?

I have just been given a very cool looking old smith rev counter. Black and chrome. rv1 1000/00a is on the front. I have hooked up the 12v + and - side. easy but the wire running to the coil has two ends. the wire runs round a coil on the back, loops round once and then goes no where. I have hooked it up and attached one end to the - of the coil and had nothing happen, I touched the other end to the motor and it just makes a kill switch. What am I doing wrong, who sells smith guages?

regards

Philip

The smiths tacho probably goes in series with the coil, so all coil current must pass through this wire.

check out this page.
http://www.caigauge.com/motorsportfit.htm

correct it mush run in series. i had a old one, but was having probelms with it effecting my spark.