Nearly all US built radio receivers made from the early days to the late 1930's required an external "long wire" antenna. Most sets made after the late 1920s had one or more terminals on the rear of the chassis or a *pair of wires hanging out the rear that was hard wired to the radio's circuitry. The colors were often black & blue, green & black or they could be any color. Black is usually the ground wire. These wires are for connecting an antenna and ground. The antenna is easily identified; if you are not sure which wire is which, the antenna will be one that makes the radio (when turned on and the volume turned up) come alive when you come in contact with the bare end with your fingers. Unless the radio has internal wiring issues or shorts, you don't have to worry about getting shocked by touching the antenna or ground wires (assuming they are in fact the correct wires I'm describing) and the radio has been properly serviced by a competent technician. If you are not sure, don't mess with it.
The ground wire is not always necessary and some radios SHOULD NEVER HAVE A GROUND WIRE CONNECTED! Such radio's are the AC/DC type and should have a warning sticker on the back of the set. If you are not sure, do not attempt to connect a ground wire. It's easy to identify a radio that is safe for a ground connection, they are the ones with a power transformer. Again, if not sure, don't connect a ground wire. The radios that are unsafe for ground wires are the low priced, low end sets with series strung tube filaments (more properly called, heaters). Such sets will often have what's called a "Hot Chassis" and a dangerous electrical shock hazard exist with such radios if certain precautions are not observed. Such radios purchased from the author of this article will have a polarized line cord and should never be defeated.
Beginning in the late 1930's, built-in antennas were introduced as part of the latest state of the art equipment for most radio models. At first they appeared on the better quality console models and within a few years were pretty much standard equipment on most all radio sets.
Most radios that require an external antenna have an "antenna" and "ground" post terminal at the chassis rear, some may have a couple of wires coming out of the chassis. A good radio working properly may pick up a station or two without an antenna connected but reception will be weak and likely very noisy. Often a 15 - 20 foot piece of small gauge bell or speaker wire strung along the baseboard will usually allow reception of stations within a 15 - 20 mile range. For better reception, a longer wire, higher in the air is necessary. I have strung an antenna in my attic that serves quite well for good all 'round reception for continental as well as worldwide short-wave reception.
* A few more things of note;
if your radio has two
loose pigtail wires coming out the back, chances are; they
are the antenna & ground wires. The ground wire is seldom needed. If you are not familiar with operating such and ancient
radio, here's some tips; you should get local stations with an
extension of a 20 -30' piece of additional wire. For long distance
reception (more than 25 miles away) a longer and higher wire will be
needed. Reception depends on the stations power, distance and the time of
day or night. One other thing, unlike modern radios, the early radios
did not have what's called "AVC" (automatic volume control). This
means weak stations require a high volume setting and strong
stations a low setting. So it's not abnormal while tuning from a
weak station to have a strong local station to come in with a BLAST!
Sonny, the Radiola Guy