Wells never revealed it ... exactly. This is from the book:
The thing the Time Traveller held in his hand was a glittering metallic framework, scarcely larger than a small clock, and very delicately made. There was ivory in it, and some transparent crystalline substance. And now I must be explicit, for this that follows--unless his explanation is to be accepted--is an absolutely unaccountable thing. He took one of the small octagonal tables that were scattered about the room, and set it in front of the fire, with two legs on the hearthrug. On this table he placed the mechanism. Then he drew up a chair, and sat down. The only other object on the table was a small shaded lamp, the bright light of which fell upon the model. There were also perhaps a dozen candles about, two in brass candlesticks upon the mantel and several in sconces, so that the room was brilliantly illuminated.
[snip]
`This little affair,' said the Time Traveller, resting his elbows upon the table and pressing his hands together above the apparatus, `is only a model. It is my plan for a machine to travel through time. You will notice that it looks singularly askew, and that there is an odd twinkling appearance about this bar, as though it was in some way unreal.' He pointed to the part with his finger. `Also, here is one little white lever, and here is another.'
Does it go 'round in a circle?
If so, ask Billy Preston.