AC Spark Plug Tester Model L

[TexRiv_63]

I know basically what this is but I'd like to know what year it was introduced and exactly what it was used for. I vaguely remember they would put your old plug on one side and a new one on the other and show you the difference in spark intensity, but what was the air connection for and what did the indicator dial measure? Thanks for any assistance.

[edinmass]

Neat item.....Phillips screws......post 1936 if memory serves me. That said, just pre or post war. Fun garage sculpture.........I’m guessing air pressure would increase resistance for jumping the spark.