Use a 50K miniature POT. RS has those too! You hook it up to the No: 6 leg of that IC. The dimple is leg 1. The count of the legs goes down and then starts back up on the other side. DO NOT go to the top on the opposite side of the IC to find leg 6. The count of the legs is like a "U".

The orange grabber is where you can measure the Vmem...

Connect the IC grabber wire to the middle leg of the POT and then depending on what direction you want to turn the pot to decrease resistance (increases Vmem) will determine which outer leg you will use for the ground wire. You can attach the ground wire to the case or m/b mounting screw; just make sure it is tight and will not come lose later on...

NOTE: Make sure you diagram on a piece of paper the direction to decrease resistance and then set the resistance to maximum before hooking it up.