You need a thermostat also commonly referred to as a tstat. You plug the uth into a tstat and the tstat then monitors the temps and reduces or adds power as needed to keep the uth at the set temp.

I use and love Herpstat. I only suggest them now as I have had two lesser tstats fail. Until you get a tstat you can use a dimmer also known as a rheostat. It is nothing more than a device that cuts the power delivered to the uth which in turn reduces the uth temp. These can be had at hardware stores for about $10.

http://spyderrobotics.com/home/products.html

Also the analog thermometers are not very good and you may check into getting a digital one. Many use the Acurite thermometer hygrometer combo.

http://www.amazon.com/AcuRite-00891A...te+thermometer