When I wanted to buy a light bulb to replace the broken one in the front lamp of my bicycle, they sent me to Duffy’s angling equipment store. I’m not good at electricity, but I managed to explain the kind of batteries and their arrangement to the old man who served me. (Voltage and amperage were indicated on the broken bulb but not on the spare ones he was selling.) Finally he found something suitable and said, “This bulb should do it. Q.e.d.” And it did do it 🙂