Anyone ever use an Amprobe?

Hey all,

 

Once upon a time, my father taught me how to use his Amprobe.  He's long
gone, but I still have his Amprobe.  I'd like to test the amperage of a
towel warmer we bought.  I separated the two wires, and placed the Amprobe
hook around one of them, but I don't get a reading.  I also couldn't find a
place for a battery.  

 

Anyone know how to use this thing?  

 

Thanks,

 

Mike