The first coil of wire is made of No. 18 insulated, and the beginning end is made fast to the binding-post at the left. The wire is carried up through the first hole under the hoop, and after three turns have been made the end is carried down through the second hole and made fast to the foot of the second binding-post. The second coil is of No. 24 insulated copper wire. The beginning end is made fast to the second binding-post, carried up through the third hole, given five turns about the hoop, drawn down through the fourth hole, and attached to the third binding-post. The third coil is of the same size wire but has ten turns. The fourth coil has twenty turns, and the fifth, of No. 30 insulated wire, has thirty turns, the last end being attached to the post at the right. In all the coils there should be a total of sixty-eight turns, or about one hundred and five feet of wire. For strong currents the in-and-out wires may be attached to posts Nos. 1 and 2 at the left, and for weaker currents to Nos. 2 and 3. For still weaker currents, use Nos. 3 and 4, and so on. To detect the very weakest currents, attach the in-and-out wires to the first and last post, and let the current travel through all the coils or the entire length of the wire wound about the hoop. The magnetic needle is made in the same manner as described for Fig. 10, and the pointer is attached in a similar fashion. But instead of being mounted on a pivot over a card, and so exposed to the open air and possible draughts, the delicate mechanism is arranged within a brass hoop, which is made fast to the ledge. The graduated card is at the bottom of the hoop, or box formed by it, and to protect the needle and prevent it from being displaced it should be covered with glass. This can be done by making a split ring of spring-brass wire and pressing it down inside the hoop. Over this a round piece of glass is placed, and another hoop is pressed in above it to hold the glass in position. If the rings are carefully made and of stout wire they will stay in place; otherwise a drop of melted sealing-wax or paraffine will be necessary to keep them where they are wanted. The glass should be arranged close enough to the needle to prevent it from jumping or being shaken off the supporting pin, but not so close as to prevent its moving easily. Part II Chapter VII ELECTRICAL RESISTANCE