5D40.10 Jacob's Ladder

PURPOSE: To demonstrate conduction in gases.

DESCRIPTION: A Jacob's ladder apparatus is demonstrated.

SETUP TIME: Medium

COMPONENT LIST:

    


  1.  Jacob's Ladder apparatus.

SUGGESTIONS: WARNING!: This is a very striking but DANGEROUS and potentially fatal demonstration. Keep the students well away and don't let them try any stunts like lighting a cigarette from the arc. Neon-sign transfomers put out 15-30 milliamps and 7ma has proved fatal to susceptible subjects.

REFERENCES: 2122 9.8: PIRA 5D40.10

OTHER NOTES: A Jacob's Ladder is nothing but a 12-15KV neon-sign transformer with slightly diverging electrodes coming from the high-voltage terminals. A spark starts at the bottom and runs to the top of the V formed by the electrodes. What is happening? Quite simple. The spark produces an envelope of heated air, which, being less dense than the ambient air, tends to rise. It does so, and carries the plasma channel of the spark along with it. The spark prefers the already existing plasma channel to making a new one lower down, even though the path length gets longer and longer...and...l-o-n-g-e-r. Finally the dischage quenches and a new spark starts at the base of the V. You can prove this by blowing down CAREFULLY on the spark; it will stay in place.