Figure 149 shows a complete telephone ready for mounting on the wall. It consists of a receiver, telephone transmitter, bell, hook, and push-button. The bell is mounted on a flat base-board. The transmitter is similar to that just described, but is built into the front of a box-like cabinet. The box is fitted with a push-button at the lower right-hand corner. A simple method of making a suitable push-button is shown in the upper left-hand part of the illustration. It consists of two small brass strips arranged so that pushing a small wooden plug projecting through the side of the cabinet will bring the two strips together and make an electrical connection. The "hook" consists of a strip of brass, pivoted at one end with a round-headed brass wood screw and provided with a small spring, so that when the receiver is taken off of the hook it will fly up and make contact with a screw, marked _C_ in the illustration. When the receiver is on the hook, its weight will draw the latter down against the screw, _D_. The hook is mounted on the base-board of the telephone, and projects through a slot cut in the side of the cabinet. Four binding-posts are mounted on the lower part of the base-board. The two marked _B_ and _B_ are for the battery. [Illustration: Fig. 149.—A Complete Telephone Instrument. Two Instruments such as this are necessary to form a simple Telephone System.] That marked _L_ is for the "line," and _G_ is for the ground connection or the return wire. [Illustration: Fig. 150.—Diagram of Connection for the Telephone Instrument shown in Fig. 149.] The diagram of the connections is shown in Figure 150. The line-wire coming from the telephone at the other station enters through the binding-post marked _L_, and then connects to the hook. The lower contact on the hook is connected to one terminal of the bell. The other terminal of the bell leads to the binding-post marked _G_, which is connected to the ground, or to the second line-wire, where two are used.