An electrical engineer and his beloved had been going steady for quite some time, so thoughts of marriage were beginning to skittishly wander through their minds. Eventually, one day, the engineer said they should make sure they were really compatible, and would it be OK if they tried something different that evening. His partner raised an eyebrow, and wondered what this would involve. "I'll bring some gel and cables.", said the engineer as he left the lunch-room, "I'll see you this evening. I'll bring a take-away"
Evening came, and he knocked at his partner's door.Upon opening it, his partner saw him carrying a small box and a take-away from their favourite restaurant. After a nice meal, the engineer said, "Right, I'll need you to take your clothes off so I can apply the gel in the appropriate places." His partner was a bit surprised as this more direct approach than normal, but wanting to see how things would turn out, off the clothes came. The engineer then stripped as well, and started applying dabs of gel in spots all over his partner's body, and his own. At each spot he applied an EEG test patch. When he had finished, his partner was feeling pretty silly, and wondered what kind of fetish this was. It seemed harmless, so far. The engineer then took out a pair of conductive wrist bands, connected by a cable, put one around his partner's right ankle, and the other around his own right ankle. He then pulled out a volt-meter from the box.
At this point, his partner lost patience and asked him what on earth he was doing.
"Oh!", said the engineer, "I thought it was obvious. I want to check if we are compatible, so..."
"I'm going to check if we have any potential differences."