While shopping with my granddaughter in a Buck Stop store (everything is $1), I found a ruler that is already gaffed to do The Ruling Class from Max Mavens The Other Month column in the July Genii. For $1 you get a plastic ruler with a battery unit on one end. When you press down on the end, the numerals light up. More importantly (for us), the weight of the battery and light makes the outcome off-center. The brand name is Fiskars, and the best part is that the ruler has inches on one edge and centimeters on the other to you can repeat the trick, writing a metric length (19 cm) the first time and using measure the pencil (which is 4 inches long) the 2nd time. (4 inches is not the same length as 19 cm)!! I have also seen rulers with a pencil sharpener on one end that probably would work the same way. Check out your local $1 store!
At the same time, I also found some ball-point pens that write in four different colors. This is great for another Maven effect, Rainbow Matrix, in the Violet Book and now in Prism. Using this you can do the matrix with only one pen and switch colors for each spectator. Brand name is Flic, and you get three pens for $1 so you can share with your friends.