I need help with Pound to Franc exchange rate in the 1800's

Yikes. I can't even imagine what resource you need, but it's a great question.

Exactly when in the 1800s?

And the pound then isn't the same as the pound now.

Then we used pounds, shillings and pence; in the 70s we went...what's the correct term? Decimal? Metric?

Easy. Do a quick search of the Old Bailey records:

www.oldbailyonline.org

Keyword: francs
Date Range: 1800-1899

Example of entries:

Grand Larceny 1824
A guy was accused of stealing:
Seven promisory notes for payment of 1000 francs each, in French money, value 41 l. 13 s. 4 d. each;
Five other promissory notes for payment of 500 francs, value 20 l. 16 s. 8 d.

Burglary 1821

Q. Before the new coinage those demi-francs were offered as sixpences
A. Yes, it is dated 1808, a great many were coined that year, I got the sample bag from Mark-lane ...

A few more searches should give you a resonable range.

Good luck!

Mac

And...

if you look in the thread stickied at the top of this board called 'Resources by Era', you will see three websites under the heading Money. The first gives you a site where you can find exactly what you want, as does the third.