The diagram in Figure 195 shows the arrangement of a simple receiving outfit. The _detector_ consists of a sensitive mineral placed between two contacts and connected so that the aerial currents must pass through it on their way to the ground. A telephone receiver is connected to the detector so that the _rectified currents_ (currents which have been changed into direct current) pass into it and produce a sound. By varying the periods during which the key is pressed at the transmitting station, according to a prearranged code, the sounds in the receiver may be made to assume an intelligible meaning. HOW TO BUILD WIRELESS INSTRUMENTS *The Aerial* Every wireless station is provided with a system of wires elevated high in the air, above all surrounding objects, the purpose of which is to radiate or intercept the electromagnetic waves, accordingly as the station is transmitting or receiving. This system of wires is, as already has been stated, called the _aerial_ or _antenna_. The arrangement of the aerial will greatly determine the efficiency and range of the apparatus. The aerial should be as long as it is reasonably possible to make it, that is from 50 to 150 feet. It will be necessary for most amateurs to put up their aerial in some one certain place, regardless of what else may be in the vicinity, but whenever possible the site selected should preferably be such that the aerial will not be in the immediate neighborhood of any tall objects, such as trees, smoke-stacks, telephone wires, etc., because such objects will interfere with the aerial and noticeably decrease the range of the station, both when transmitting and receiving. Bare copper wire makes the best aerials. Aluminum wire is very often used and on account of its light weight causes very little strain on the poles or cross arms. Iron wire should never be used for an aerial, even if galvanized or tinned, because it tends to choke the currents which must flow up and down the aerial when the station is in operation. [Illustration: Fig. 196.—Molded Aerial Insulator] The aerial must be very carefully insulated from its supports and all surrounding objects. The insulation must be strong enough to hold the weight of the aerial and able to withstand any strain caused by storms. Special aerial insulators made of molded insulating material and having an iron ring imbedded in each end are the best. [Illustration: Fig. 197.—A Porcelain Cleat will make a Good Insulator for Small Aerials.] Ordinary porcelain cleats may be used on small aerials where the strain is light. One insulator should be placed at each end of each wire close to the spreader or spar. Most aerials are made up of four wires. The wires should be placed as far apart as possible. There are several different forms of aerials, the principal ones of which are shown in Figure 199. They are known as the grid, “V," inverted “L,” and “T” types.