From the day the telephone was invented with Graham Bell’s patent in the late 19th century until the first decades of the 20th century, the devices were mainly made of wood. The telephones of that time were mostly wall-mounted. They were bulky. And they had a built-in manual magnet which, when rotated, produced alternating current to notify the operator at the telephone exchange and to activate the ringing circuit in all the devices that were on the same line. There was also space in the device for a battery. The telephone exchanges of that time did not have central batteries, so there were local batteries in the telephones that consisted of dry zinc-carbon cells.