Guest 2434 portable corrosion meter with leads for measuring stray galvanic corrosion in your boat and to determine whether you have enough zincs installed to take care of the problem.
New the unit sold for $246 but mine is a couple of years old or so but still works perfectly.
The procedure is to drop the silver anode into the water around your boat closest to the item you wish to measure for galvanic action.
The other lead gets attached to the item being measured like a bronze thru-hull or prop shaft or whatever and then read the millivolts on the meter scale.
The meter scale is graduated for each metal - aluminum; steel; or bronze and the color code on the scale will advise you if you are protected or not.
I am selling the complete meter kit for only US$100.