how to make a vectorField2D dynamically selectable?

currently i can make it show if i go inside Ejs and change n = 11 to n =30.

but i can't remember and i can't find previous posting on this
can ask how to make a vectorField2D dynamically selectable?

want to allow students to vary the visual vectorField2D to see that it is just a representation like in the 2 attached picture.

thanks!
trying to make a better version of this http://webphysics.davidson.edu/physlet_resources/bu_semester1/c17_field.html http://webphysics.davidson.edu/physlet_resources/bu_semester1/index.html in Ejs

/htdocs/ntnujava/ejsuser/14019/users/sgeducation/lookang/gravityfield_pkg/gravityfield.propertiesFull screen applet or Problem viewing java?Add http://www.phy.ntnu.edu.tw/ to exception site list
Press the Alt key and the left mouse button to drag the applet off the browser and onto the desktop. This work is licensed under a Creative Commons Attribution 2.5 Taiwan License
Download EJS jar file(953.6kB):double click downloaded file to run it. (4 times by 2 users) , Download EJS source View EJS source