I have an Airguide barometer with a sticker on the back showing I- 1795 or 1- 1795, presumably a model number? The front of it says "percent relative humidity. Haven't looked at it for 5 yrs. Recently moved to a condo that seems too dry; so I set it out again. Have moved it to three different rooms over a period of three days. Needle never moves. Discovered for the first time that there is a thing in the back that will move the needle. Why is it needed? I thought the needle was supposed to move by itself according to the humidity in its location. It always did in the past. How old is it?