What is this adjustment?

[Scotty]

Hi All -
I have been learning so much from this forum - I owe you all a pile of thanks. I just recently painted a room in my home and to do so I removed the thermostat for my Lennox G12 from the wall. My youngster got a hold of it by mistake. When I got it away from him the outer panel had been removed. Below the mercury switch is a round disk with an adjustable needle, The disc says "long and short" with arrows. I am concerned this was moved, but I have no idea what it does/how to set it. Can you folks help?
Thanks in advance!
Scott S
Denver, CO

[Hube]

Scotty; I assume this is the "anticipator. it can control the length of time the systems heat runs. Setting it properly is usually done with an ohm / ammeter tester. The setting is usually .45 for gas fired and .20 for oil fired.Check to see if the setting is marked on the thermostat case. You can set it mid way and run the system and note the length of time the burner is on and how much thermostat "override occurs (if any) When you set the therm for 72, the anticipator should shut the burner down approx at 71 to allow for heat build-up. Hope this helps, Hube