Wheatstone bridge problem

I've used a Wheatstone Bridge to measure the resistance of 4 large solenoids. The readings were: 0.3, 0.28, 0.27, 0.26 ohms.
I measured all of them again and then they were all about 0.20 ohms.
As you can see, the readings have slowly dropped from 0.3 to 0.2 ohms.
Can anyone offer suggestions why the readings keep changing?
I assume that the battery condition is independent of the measurements.

Each time I did a measurement I reconnected the alligator clips to the terminals and wriggled them around to get a good contact, and checked the reading three times for each solenoid. Each of those three readings was the same. i.e. I've done a total of 24 readings.

When I connect the two leads from the Bridge together I get a reading of 0.05 ohms. There appears to be several hundred feet of 14 gauge wire on each solenoid. Each time I press the check button on the bridge the meter needle swings slightly to the left of its final position. Each solenoid weighs about 20kg. There is no way they can be changing temperature!

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