Science Buddies: "Ask an Expert"

Hi

For one of my science projects, it is proposed that a spiral metal spring will experience a small contraction if an electric current is passed through it. I would like to know how to design an experiment to investigate this effect (I have to include a diagram showing the arrangement of the apparatus and a description of the procedure to be followed). I must also pay special attention to the stiffness of the spring that is to be used, the magnitude of the current to be passed through the spring and the method of measuring the contraction of the spring (given that it is likely to be very small). I would appreciate any help with this project.

Thank you,

Darien

This should be a very interesting project.

One variable you did not mention is the temperature of the spring. As you pass current through it, it is possible you will increase its temperature. This will cause a change in the spring's dimensions, and affect your result. So be sure to take this into consideration.

Measuring the amount of contraction is probably your biggest challenge. What does your hypothesis tell you to expect for amount of contraction? This will help you select an appropriate method to measure the motion. Clearly, if you predict a large movement, a ruler or digital caliper would work. If it is small, you might be able to use a dial indicator. If you expect motion on the order of a couple thousandths of an inch or so, it get really tough. Measuring thousandths of an inch aren't hard, but most methods require contact with the thing you are measuring, which, in your case, would probably change the deformation of the spring.

So you'll need a non-contact measurement method. There are a number of laser-based devices that could do this, but they may be too expensive for you. You should check with your school to see if they have one. Also, if the spring is sturdy enough to attach a small mirror to it, you could make your own interferometer. Here's some information to give you and ideal of how to do that:


http://en.wikipedia.org/wiki/Interferometer

http://opticsnotes.nixsar.com/scioptics ... ometry.htm

http://www.metacafe.com/watch/1381543/l ... de_for_20/

http://www.ligo-wa.caltech.edu/teachers ... r_9t12.pdf

Good luck, and have fun!