John W. Dewdney

Energy Loss from the Filament of an Incandescent Lamp

The current-voltage characteristic of a lightbulb can be analyzed graphically to reveal much about heat loss from a hot filament. The low-temperature region is particularly interesting. The experiment can be done and the effects understood by elementary students.

Poor Man's Mass Spectrometer

A mass spectrometer has been designed around simple constituents: the vacuum system is made from copper plumbing parts; electrical leads into the vacuum are via darning needles pushed through rubber stoppers; the filament of the thermal ionization source is from a small light bulb; object and image slits are made from bits of razor blades sandwiched between brass washers. The magnetic field is supplied by a permanent magnet fitted with homemade cylindrical pole pieces (of such a diameter to give second-order direction focusing). A modest vacuum of 10−4 mm Hg is required. When the instrument is focused (by adjusting the position of the pole pieces) and aligned (by rotating greased rubber stoppers) a resolving power of 50 can be obtained.

Apparatus Review: Open Air Alpha Particle Counter

The counter detects alpha passage by utilizing the dielectric breakdown of air subjected to a high potential gradient. Breakdown occurs whenever an alpha particle ionizes a path in the air between a wire grid and a plate. A small, visible spark jumps between the grid and plate, thus indicating the alpha passage. Negative pulses of 5000 v and 10 μ -sec duration may be obtained. The discrete nature of radioactive emissions may be demonstrated; with modifications, the device may be used for more quantitative observations.

Domino Rule. A Way of Handling Relative Velocities

Relative velocities can be combined in a way similar to the positioning of dominoes in the parlor game. The advantage of this method over the more usual one involving addition or subtraction of vectors is that it is more intuitive. It is particularly useful in the transition between laboratory and center of mass coordinate systems.

Undergraduate Mass Spectrometer

Described here is a simple static-field mass spectrometer which has had usefulness both as an undergraduate laboratory instrument and as a piece of demonstration apparatus. The design provides for the adjustment of such parameters as magnet position, slit widths and heights, and allows the use of different types of sources and detectors. Ion currents are large enough to be easily measured or displayed on an oscilloscope.

Graphical Method for Comparing Galvanometer Sensitivities

A method is described for making graphical comparisons of galvanometers of different resistances and periods. The effective sensitivities in a given circuit can also be easily compared.

Improvement to the Franck-Hertz Experiment

Bernheim, Gossard, and Pound have devised a modification of the Franck-Hertz experiment in which the traditional peaks are displayed on an oscilloscope. Minor sophistications in their method make possible the display of 25 or more maxima.