Best way to let users enter 'infinity' in a html form

Question

I have a form with a field "quantity" which can have the value "unlimited / infinity":

Quantity: [ ]

What would be a good way to let users enter "infinity"?

Some ideas:

1) Tell the user that -1 means infinity:

Quantity: [ ] (-1 = unlimited)

2) Tell the user that a blank field means infinity:

Quantity: [ ] (blank = unlimited)

3) Radio buttons

() Quantity: [ ]  or  () unlimited

Any ideas? Or links to related usability studies?

Answer 1

I prefer option #3 as it does not require the user to do anything special such as entering a special number or character. This makes it more difficult for the user to make a mistake. It also is easy to handle programmatically and, assuming you name your fields and variables appropriately, make your code cleaner and easy to maintain since the special value is separated from the "normal" values.