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The Hodoscope

After writing a post about the hodograph of Sir W. R. Hamilton and how it relates along with a few other rather obscure mathematical theorems to angular momentum and planetary orbits, my wife, a PhD physicist, remarked, “Yes, but what about the hodoscope?”.  A little bit of digging revealed that unbeknownst to me she had actually built a hodoscope.

A hodoscope is a subatomic particle detector that displays the path a particle travels along through a region of space.  It does this by utilizing an array of single particle detectors that each on it’s own can only detect the presence of a particle in its active region.  By tracking the binary events produced by each detector, (logical high for the presence of a particle and logical low for its absence), the hodoscope can reconstruct the track of the particle display it on an array of pixels on your computer screen for example, or on an array of light bulbs, as in the original hodograph discussed below.

The first hodoscope, was designed and built by Dr. Thomas H. Johnson and Dr. E. C. Stevenson of the Bartol Research Foundation of the Franklin Instititue.  In time for the 1933 World’s Fair in Chicago. The Review of Scientific Instruments described it as:

… a hodoscope, designed by Dr. Johnson of the Bartol Institute, will show the paths of individual cosmic rays by means of flashing neon lamps. The astonishing thing about these rays is that they have come to us through our atmosphere, which is equivalent to over two feet of lead, whereas similar radium rays will penetrate only about two inches of lead.

A diagram of the array of detector tubes used in the hodoscope is shown below. All diagrams are taken from the Franklin Institute report on the device.

M1 and M2 are detectors that are used to determine when a single particle has penetrated the entire device as a cosmic ray would, but a lower energy background particle from the laboratory would not.  If M1 and M2 both fire at nearly the same time, then the tubes in the array are allowed to fire and light their constituent bulbs in the devices display.

Why should all of this be interesting to electrical engineers?  Check out the circuit that detects coincident events between M1 and M2 and uses those events to enable the array of tubes to display the cosmic ray’s track.

It’s an early example of a vacuum tube logic circuit!  Notice that they could incorporate a number of gates, (called grids in vacuum tube parlance), denoted by the squiggly lines in each tube.  The operation of the circuit is described in one final excerpt from the Franklin Institute’s report:

In closing, I leave you with a picture of the hodoscope from Electrical Engineering magazine circa 1933 and it’s eerily similar modern counterpart, (in form, but not in function), the Fringe light box.


1.  Franklin Institute Report

2.  IEEE Electrical Engineering Article

3.  Review of Scientific Instruments on the 1933 World’s Fair

4.  hodagraph post

5.  Built a hodoscope


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