Q. In the schematic symbol for a Capacitor, one plate is straight and the other is curved. Today, the curved plate in the symbol for a non-polarized Capacitor has no significance. But at one time, it did mean something. Can anyone tell us what it was, and how you could tell which plate it was by looking at the actual capacitor?
A. The curved plate was the outside foil. On the capacitor, it was the end marked with a black band.Refer to figure 1. (attached)
A paper capacitor was made by taking a sheet of (very thin) aluminum foil, a sheet of paper for the dielectric, another sheet of foil, and rolling them up into a tube. A lead was attached to each plate, and the roll was covered with wax-impregnated paper. They were popular in tube equipment high working voltage and low Capacitance.
Today, a replacement would be the Film capacitors. These capacitors, also known as plastic film capacitors, film dielectric capacitors, or polymer film capacitors, are generically called “film caps.” This type, as well as power film capacitors, are electrical capacitors with an insulating plastic film as the dielectric, and sometimes are combined with paper (like mentioned above) as carrier of the electrodes. To illustrate the difference, please see figure 2. (attached)