Speedcolor Marc I - How do I use it?

I happen to get a hold of a Speedcolor Marc I Color/Density Analyzer for my darkroom but it doesn't come with instructions. Does anyone out there know how to use it or know where I can get a manual? I was just going to use it for B&W printing & I heard that it can determine the exposure time fairly accurately without wasting test strips. It has this round magnetic end a little smaller than a quarter & I think I'm supposed to put it under the negative & get a density reading. I see the needle moving when I place the sensor(?) around the dark/light areas on the negative but I'm not sure how to interpret the readout.

Thanks in advance.