In general, you can say, direct current = short distance and alternating current = long distance. Tesla didn't give up on direct current by any means. He recognized its value in many short-range applications. Today the insides of almost all computers and other electronic devices use direct current exclusively. The methods shown in this patent are still good today, and are always worth another look.
Whenever you open up any contemporary electronic device, including radio, TV, computer, or appliance, the first thing you see after the power cord enters the unit is a "power supply". In most cases it is in a special metal cage. If you open up the cage you will usually find some kind of transformer. You might find some large capacitors too — those will look a bit like cans. There may also be rectifiers and diodes which convert the current from AC to a low voltage DC. What's inside the cage is generally simple and often easy to repair. That's why so many technicians are happy when they know the problem is no power at all — it's usually just inside that cage.
Many units use a standard of 5 volts DC, although there are variations. Then, everything outside the cage will be DC.
That doesn't mean the DC circuitry beyond the power supply cage is safe by any means. Enough current can be stored in capacitors within a DC circuit to produce a pretty heavy jolt, so make sure you know what capacitors look like and get them discharged before doing anything else!