How to use sforce.connection.partnerServerUrls

I’m stuck! Can I get an example on how to properly use sforce.connection.partnerServerUrls.?

The documentation and a user group post appears to say I can call sforce.connection.partnerServerUrls.16

I have also tried:

var num = 8;
var n = num.toString();
var partnerServer = sforce.connection.partnerServerUrls.n;
url=…+partnerServer+…

But I get undefined.

Thanks!