Full Version: Polarity of my special
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Many years ago i purchased a Cambridge special which was a non runner. the intention was to have it running in a short period of time. This never happened and many years have rolled by. I am now however determined to get it on the road. ( whilst I am still able to get in and out of it) I have however come across two problems which have got me stumped,they are as follows. As I bought it as a non runner and without a battery I do not know if it is positive or negative earth or indeed has a previous owner changed it from 6 volt to 12 volt. The "big red book" really does help that much as the year of manufacture is 1936 and there were lots of changes about that time also a lot of people made the switch from 6 to 12 volt. 
Is there a sure fire way to answer these questions with the limited information I have been able to offer. Thank you in advance
Regarding polarity, decide which you prefer and set it up that way. All you have to do is connect the battery the way round that you want, then flick a wire from the non-earth side of the battery onto the F (field) connection on the dynamo. The ammeter might read the wrong way around, so swap the wires on it if you want.

6 volt or 12 volt is nearly as easy. fitting of a battery of the voltage you prefer is the main thing. Then somehow preventing too much current flowing through the dynamo field windings.
Assuming it has lights fitted remove a bulb and check the voltage stamped on it. That should give a clue
The positive and negative battery terminals are different sizes. That will tell you what polarity it was set to. The +ive I think is the larger.