watching variables in a service

[Garey Mills]

I have a service that returns an object that has a single variable, say w, with some numeric value. I want to 

build a directive that publishes that value in an HTML page. Using a link function in the directive, I tried this:

 $watch(variableService.w, numToPublish,

   function(variableService.w, numToPublish) {

       numToPublish = variableService.w;

       element.appendChild(numToPublish);

   });

but I get: Uncaught SyntaxError: Unexpected token '.'

I've also tried square brackets, as in variableService['w']. Chrome developerTools console that the square bracket is the unexpected token.

Is there some recipe that will allow me to watch variables on a service?

Thanks