A fuse is a device designed to break a circuit, usually by melting when the current exceeds a certain value. Fuses are widely used in electronic equipment, but have been replaced by circuit breakers in household wiring. In the "old days" people would sometimes replace a blown fuse with a penny, which happened to be the same size as a fuse. Was this a safe practice? Why

High-voltage power supplies are sometimes designed to have a rather large internal resistance as a safety precaution. Why is such a power supply with a large internal resistance safer than one with the same voltage, but lower internal resistance?

Can all combinations of resistors be reduced to series and parallel combinations?

Old-time Christmas tree lights had the property that, when one bulb burned out, all the lights went out. How are these lights connected, in series or in parallel? How could you rewire them to prevent all the lights from going out when one of them burned out?