Likle a radiator, but involves the use of a cat. You plant your feline on the radiator, uit warms up (making it very happy), then you transfer the warmed cat to your lap; reap the benefits, then replace the cat on the radiator. Repeat the cycle as necessary.
I was so cold last night, that I stuck the pusskat on the rad in the front room for 10 mins, and Hey Presto! fully-charged Catdiator! My lap was comfily warm for the rest of the evening.