Using substring?

Field One is a number and then a name of a unit.  Example:  TR 987 Tree Patterns

I want to auto-populate Field Two with information from Field One--but just the first 7 characters.

In Field One--I placed the following code under In Blur->Run JavaScript

//Set the vars one and two

var one = this.getField("FieldOne");

var two = this.getField("FieldTwo");

{two.value=one.substring(0,7)}

It does nothing. 

Any ideas?