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Lecture: Hair Dryer Spectrograph

Make them respect the mass spec.

Jon and I both thought that knowing a little about the mass spectrograph was needed in order for our students to have real appreciation for it’s role in demonstrating the existence of isotopes. The truth be known I have never even seen one, but Jon had used the mass spec many times in his doctoral work on the DDT levels in the ice layers in Alaska. The point however is that we wanted students to understand a bit about how the thing worked and why it was so instrumental in work related to isotopes. We also knew that discussions of bubble chamber tracks would also be easer for our students if we could get them up to speed on the mass spec.

The idea started with trying to bend the path of a moving ball by blowing air on it at a right angle to it’s motion. A hair dryer became the instrument of choice to blow a moving ball into an arc that would be part of a circle. On a table top covered white paper and a few eight and a half by eleven sheets of carbon paper place carbon side down and using a heavy ball we could get a trace of the partial circle formed. Some practice was required in order to move the hair dryer along staying at a right angle to it’s motion and the same distance off to the side of the ball. Sliding the nozzle of the hair dryer along the table just to the side of the ball gave us nice circular arcs that were convincing and even measurable with a large black board compass. We gave the ball a start by rolling it down a ramp and if we were going to compare two different balls we would run a race to make sure we were starting them from the proper point on the ramp to get equal velocities.

Over time we collected three balls (steely, woody and a brassy) that were the same size, but different in mass. The woody was far less massive and would give a small arc relative to the other two. The steely and brassy were much closer, but never the less the smaller arc for the brassy was very much evident. We would later suggest that they might be a little like a pair of isotopes. We searched for and found a lead ball that had the same mass as a steely and after adjusting the velocities the lead sphere would give a much larger arc bringing out clearly the issue of surface area for the wind to grip. In just a few minutes students were up to speed on the idea that mass and velocity made the arcs larger while wind strength and surface area made them smaller. The jump to seeing surface area as the charge factor on an ion and wind as the strength of the magnetic field is not difficult. If we hold the velocities the same and hold the magnetic field strength steady the only variables are the charge on the particle and thats particles mass. Without much further ado the picture often found in chemistry texts comes to life and the issue of isotopes seemed easy along with the need to announce a new subatomic particle equal in mass to the proton, but having no charge. Just for fun we would have the class watch a ping-pong ball moving in our hairdryer mass spec. Of course it is impossible to keep the hairdryer in the proper position but we would talk about the behavior of electrons in the mass spec adjusted for ions that would be several thousand time more massive. Of course the electron would bend in the opposite direction as will be important later when we talk about bubble chamber trails.