JavaScript Numeric field Validation

function validate_Numeric()
{
//Author: Surendra Sambana
//Purpose: This function validates a Numeric field
var num = document.getElementById('Numeri Field ID');
var len = num.length;
var i = 0;
while(i< len)
{
i++;
if(num.charAt(i)<=0 || str.charAt(i)>=9)
{
alert("Please enter a valid numeric");
}
}
}