Lamp holder A connected to bulbs b, c, and d does not light up, A connected to bulb a lights up, bulbs a, b, c, d connected to lamp holder B lights up. What's the explanation for why A b, c, and d do

Lamp holder A connected to bulbs b, c, and d does not light up, A connected to bulb a lights up, bulbs a, b, c, d connected to lamp holder B lights up. What's the explanation for why A b, c, and d don't light up?

This problem is actually very common, but the "engineers" in my company spent several hours and failed to come up with the result: the dimensions of the spiral lamp base bulbs are somewhat different, and the height of the contact reed in the center of the lamp base is also different. There will be a difference. If the central contact of lamp holder A is at a lower position and bulb A cannot be fully rotated to turn it on, the bulb will not light up. However, if the central contact spring of lamp holder B is at a higher position, If the bulb a can touch the contact piece normally, the bulb will work normally. Pull up the reed in the middle of the spiral lamp holder A a little and it will be OK.