Hi Mel, i have a machine that is very similar to a T.E.N.S, it was really made for use on acupuncture points but it does the exact same thing as a T.E.N.S machine, runs on 4 small batteries, no gel with this one, but i just wet the skin with a tiny bit of water were the electrode was going under the electrode then secured it on with some adhesive tape, it made the pulse a lot stronger.
I used it on my feet, it helped me with the burning feeling whilst i had it on and for a while after too and i agree with Bob 100 % in that starting on a very low setting is only and safest way to go, i think its sorta trial and error until you get the right strength for the individual.
I hope it helps him.
all the best,
Brian