Two upright pieces of wood, five inches long, half an inch thick, and one inch in width, are screwed fast to the rear edges of the base-block to support the hoop on which the insulated wire is wound. Be careful not to use any iron or steel in the construction of this or any other recording instrument, except where it is expressly stated. Screws, nails, staples, or any bits of anchoring wire should be of copper or brass. String, thread, or silk may be used, especially where coils of wire are to be bound or fastened to hoops or base-blocks. The balance of the indicating needle is so delicate, and the sensitiveness of the coils is so easily affected, that nothing about or near the instruments should be of iron or steel. The hoop may be made of very thin hickory wood, steamed and bent so as to form a ring six inches outside diameter and one inch wide. It is even possible to construct a satisfactory hoop from a ribbon of brown paper, rolled and lapped, the several thicknesses being glued as the turns are made. If a metal hoop is to be used, solder the ends of a thin, hard ribbon of brass, copper, or zinc. This strip should be provided with holes, set in pairs about four inches apart, all around the hoop, and where the hoop is to be attached to the uprights two holes should be made close to the margins through which brass screws may pass. Across the middle of the hoop a strip of wood six inches long, an inch wide, and a quarter of an inch thick is made fast. On this the graduated card is placed, and at the centre the balanced magnetic needle is arranged on a pivot. After the cross-stick is in place, wind five turns of No. 24 insulated copper wire about the hoop, keeping it as nearly in the centre as possible. One end of the wire (the beginning) is to be attached to the first binding-post on the front of the base, and the other end to the second post. The wire should be wound round the hoop in the same direction as the clock hands travel about a dial. Another coil, composed of ten turns of wire, is made over the first one, the beginning end being attached to the middle binding-post and the last end to the third post. This arrangement is shown in Fig. 11, D and E representing the coils, while A, B, and C are the binding-posts. The current enters at A, passes through coil D, and out at post B. The next passage is in at B, through E, and out at C. A current passing in at A will travel to B, thence through E, and out at C. If the leading-in wire is made fast to A, and the out wire to C, the current will travel through the entire coil. Under this plan one or both coils may be used (the short or long one as desired) by making connections with the first and second binding-posts, the second and third, or the first and third, as the strength of the current will warrant.