If a telephone receiver is used, place it to the ear. If a galvanometer is used instead, watch the needle carefully. Then move the sharp edge of the knife-contact over the scale along the German-silver "slide wire" until a point is reached when there is no deflection of the needle or no sound in the telephone receiver. If this point lies very far on one side or the other of the center division on the scale, substitute the next higher or lower known resistance spool until the point falls as near as possible to the center of the scale. When this point is found, note the reading on the scale carefully. Now comes the hardest part. Almost all my readers have no doubt progressed far enough in arithmetic to be able to carry on the following simple calculation in proportion which must be made in order to find out the resistance of the unknown coil. The unknown resistance, connected to _B_, bears the same ratio to the known coil, at _A_, that the number of divisions between the knife-contact and the right-hand end of the scale (lower row of figures) bears to the number of divisions between the knife-edge and the left-hand end of the scale (upper row of figures). We will suppose that a 5-ohm coil was used at _A_ in a test, and the needle of the galvanometer stopped swinging when the knife-contact rested on the 60th division from the left-hand end, or on the 40th from the right. Then, in order to find the value of the unknown resistance at _B_, it is simply necessary to multiply the standard resistance at _A_ by the number of left-hand divisions and divide the product by the number of right-hand divisions. The answer will be the resistance of _B_ in ohms. The calculation in this case would be as follows: 5 X 40 = 200 200/60 = 3.33 ohms 3.33 ohms is the resistance of _B_. This explanation may seem very long and complex, but if you will study it carefully you will find it to be very simple. When once you master it, you will be enabled to make many measurements of resistance which will add greatly to the interest and value of your experiments. CHAPTER IX BELLS, ALARMS, AND ANNUNCIATORS An electric bell may be bought almost anywhere for twenty-five cents, and from the standpoint of economy it does not pay to build one. A bell is not a hard thing to construct, and the time and money spent will be amply repaid by the more intimate knowledge of this useful piece of apparatus which will be gained by constructing it. The base is four inches wide and five and one-half inches long. The magnets consist of two machine bolts, wound with No. 22 cotton-covered magnet wire. Fiber ends are fitted on the bolts to hold the wire in place. The wire is wound on each of the magnets separately. Cover the cores with two or three layers of paper before winding on the wire. The ends of the wire are led through holes in the core ends. The ends of the bolts are passed through the yoke, and the nuts applied to hold them in place.