Perhaps a little history will help...
Here in America, back in the ancient past (April 1861), there was (briefly) a postal letter-delivery system that basically consisted of a chain of outpost stations in the middle of nowhere, each with a corral of fresh ponies (horses). A rider with a pouch of letters for delivery would hop on a horse at the beginning outpost station and ride like hell to the next outpost station, where he would change to a fresh horse (or pony) and continue on his journey to the next outpost station. This was repeated about every ten miles for a total ride of about one hundred miles, whereupon a fresh rider was handed the mail pouch and continued the journey. All this riding was conducted through sometimes hostile Indian territory and with the possibility of encountering other miscreants intent on robbing and/or killing the rider. It was called the
Pony Express. It lasted until October 1862 and could deliver a letter in about ten days from Missouri to California. The Pony Express was eclipsed and soon went out of business when the first Intercontinental Telegraph was completed. It was expensive to send a letter by telegram (they charged per word!) but even more expensive to a send a letter by Pony Express!
Other than the wages paid to telegraph operators at each end of the telegraph line, the most capital intensive part of a telegraph line was the copper wire strung on glass insulators between telegraph posts. This was bare copper wire, hence the need for the glass insulators, and the posts kept the wire high enough above the ground so it wouldn't get snagged by passers by.on horses. However, every electrical circuit (which a telegraph is) requires two wires connected to a source of electrical power, such as a rechargeable battery. Running a pair of wires over intercontinental distances for just one telegraph circuit would have been prohibitively expensive, but it was soon discovered that the Earth itself was sufficiently conductive to serve as the second wire of the complete circuit. This wire naturally became known as the "ground" wire since it was connected to a copper or steel rod driven into the ground at each telegraph station. Hence the origin of "Earth ground" for any electrical connection that returns to the Earth.
There is more to the story than what I have described here. Sometimes Earth grounds are used to carry significant amounts of current to deliver energy to electrical loads. The amount of power involved can be just a few watts to hundreds of kilowatts. If more than one Earth ground is involved, the corresponding currents can cause a potential difference between two Earth grounds, sometimes large enough to be felt. This is particularly a problem with dairy farms where "faults" of this nature can cause milk cows to avoid the milking barn for fear of getting shocked. The electrical utility often could care less about solving the problem.
