The force sensor that I have used is the Pasco science workshop sensor which has now been replaced by the Pasport sensor shown below. The spring must be such that a it will stretch about 5cm before the sensor goes off scale, this isn"t normally a problem since the sensor will measure up to 50N. I bought a box of 100 assorted springs from ebay for not much money. I don"t provide any clamps at the beginning of the lesson...