CHAPTER XIV.
CONSTRUCTION AND USE OF APPARATUS.
=Note.=--Before taking up the study of cells and the electric current, let us perform a few experiments in order to understand the construction and use of some of the apparatus needed for such study. A dry cell will be used as the source of the electricity for these first experiments, because it is convenient. You will understand its action later. Use this cell only as directed; improper use of it might spoil it.
=EXPERIMENT 101. To study the effect of the electric current upon the magnetic needle.=
_Apparatus._ A compass (No. 18); a dry cell, D C (No. 51); wires with spring connectors attached (§ 226) for making connections. Fig. 66 shows a plan or top view of the arrangement. Any other form of cell will do in place of D C.
=226. Electrical Connections.= One must constantly join wires, connect wires with apparatus, or connect one piece of apparatus to another, to make the proper electrical connections. A very simple method of connections has been used in all the apparatus described in these experiments.
A little arrangement which we shall call a spring connector, S C, (Fig. 61), gives us a means of quickly making connections; that is, it does away with expensive binding-posts. It is made of brass, nickel plated, and may be used anywhere without affecting the magnetic needle.
Six or eight wires, about No. 24 gauge, each about 1-1/2 ft. long, should be prepared with a connector at each end. You may use wire furnished (No. 53). Scrape the insulation from the ends of the wires for about 1-1/2 inches, then twist the bare ends around the connectors as shown in Fig. 61. The wire should pass around tightly at least 4 or 5 times and then be twisted a little, as shown, to help tighten it. Do not put it on so poorly or in such quantity that the part, B, will spread.
=227.= Fig. 62 shows how the connector should be slipped upon a thin piece of metal, M, like that on the galvanoscope, for example. The wire, W, from the apparatus itself is permanently fastened under the head of the screw, S, while the wire from any other apparatus is one of those kept on hand as above mentioned and connected with S C.
=228.= Fig. 63 shows how several wires may be quickly joined, electrically, by slipping the connectors at their ends upon a thin metal plate, M P, which may be a piece of tin, zinc, copper, etc. M P should not be too thick. In case the connectors become too much spread to pinch the plate, squeeze the part, A, a little more together.
=229.= Fig. 64 shows the method of connecting with the special form of binding-post used, for example, upon the resistance coil, R C. The end, C, of S C, is pressed down into the tube, T, until you feel, by moving it, that it springs firmly against the sides of the tube. In case you wish S C to fit very tightly in T, one of the legs may be slightly bent outwards.
=230.= The connector may be used in still another way; that is, by pushing the part, A, into the hole of an ordinary binding-post, (Fig. 65), and using it just the same as a thick wire.
=231. Directions.= (A) Stand the compass and D C near each other (Fig. 66). Attach one end of an insulated copper wire, C W, to the binding-post, C, which is on the carbon plate of the cell. Do _not_ join the other end to the other binding-post, Zn, of the zinc plate.
(B) With the left hand hold C W above and near the compass-needle, and in the N and S line, so that it will extend over the entire length of the needle.
(C) Take the free end of C W in the right hand and touch binding-post, Zn, for an instant only, watching the needle. Repeat.
_=232. Current Detectors.=_ We know that a magnet can act, by induction, through the air upon a piece of iron or upon another magnet. The deflection of the needle in this experiment shows that there must be a magnetic field around a wire carrying a current. This fact is of the greatest possible importance. The simple magnetic needle, when used as above, becomes a detector of electricity.
=EXPERIMENT 102. To study the construction and use of a simple "key."=
_Apparatus._ A key, K (No. 55) (§ 233); a dry cell, D C, (No. 51); a compass, O C (No. 18). _Arrange_ as shown in Fig. 67, which is a top view or plan. Connect the pieces of apparatus with wires and spring connectors (§ 226). Binding-post, C, is joined to I (in) of the key; O (out) of key is joined to binding-post, Zn, the wire, C W, passing directly over and near O C, which is to be used as a detector. The current cannot pass until the lever, L, is pressed. A metal plate, M P, is used to connect two short wires (§ 228) in case C W is not long enough.
=233. A key= is merely a piece of apparatus by which the circuit can be conveniently and rapidly opened and closed at the will of the operator; that is, by it the electricity can be quickly turned on or off. Fig. 68 shows a simple form of key. To the base, B, are fastened two metal pieces or straps, the upper one, L, being the lever or key proper. The front end of L is raised above O, so that the two do not touch each other unless L is firmly pressed down. A screw, S, keeps L from springing too far above O. For convenience we shall suppose that the wire leading to the key joins it at I (in); the wire _from_ the key is joined to O (out), by means of connectors (§ 226).
The key may be put into any circuit by first cutting a wire and then joining the ends to I and O. Spring connectors make the best connections with this form of key. (For Home-Made Keys see Apparatus Book.)
=234. Directions.= (A) The magnetic needle being directly under the wire, press L down for an _instant only_ and note the action of the needle.
(B) Press L again, hold it down for 3 seconds, not over that, and watch the needle.
_=Discussion.=_ The key allows us to easily regulate the length of time during which the current passes. This experiment shows, also, that the magnetic field about the wire disappears as soon as the current ceases to pass.
=EXPERIMENT 103. To study the construction and use of a simple "current reverser."=
_Apparatus._ A dry cell, D C (No. 51); a compass, O C (No. 18); a current reverser, C R (No. 57) (See § 235); an insulated copper wire, C W, 2 or 3 feet long, with spring connectors joined to its ends (§ 226).
_Arrange_ as in Fig. 70. The wire, C W, leading from X should be held by the left hand so that it will be just above (or below) and parallel to the magnetic needle.
The current cannot pass through C W until one of the straps or levers on C R is pressed. (See Apparatus Book for Home-Made Reversers.)
=235. The Current Reverser.= (No. 57.) To the wooden base (Fig. 69) are fastened four metal straps, each turned up at the end so that spring connectors (§ 227) can be slipped on to make electric connections with other pieces of apparatus.
Suppose that at C and Z connections are made with the carbon and zinc of the cell, by means of wires and spring connectors (§ 226). The current comes from the cell to C. As the two straps, 2 and 3, press firmly up against strap 4, and do not touch 1, it is evident that no current can pass from 1 to 2 or to 3 until they are pressed down upon 1. Two wires are joined by spring connectors to 2 and 3 at their turned up ends, X and Y, and these wires lead to any desired instrument.
=236. Directions.= (A) Press down lever 2 (Fig. 70), for an instant only, at the same time noting carefully in which direction the N pole of the needle is deflected.
(B) After allowing lever 2 to spring up again, and after the needle comes to rest, press down lever 3 for an instant, watching the needle. Is the N pole of the needle deflected in the same direction as it was in (A)?
_=237. Discussion.=_ The reverser gives us a quick and easy means of reversing the current which is to pass through any desired instrument, first in one direction and then in the opposite direction. Suppose (Fig. 69) that the current enters C R at C, as it does when C is joined to the carbon of the cell; the current can go no farther until one lever is lowered. If lever 2 (Fig. 69) be now pressed down, as in part (A), the current will pass along 2, which does not now touch 4, out through X to a coil of wire or any instrument, and back to the reverser by the wire joined to Y. It will then pass from 3 onto 4, to Z, and back to the Cell; that is, the current enters C W at X. When lever 3 is pressed, the current still entering C R at C, the electricity will pass onto 3 and out at Y, and back through X, 4 and Z to the cell. The current, then, can be made to pass out of X or Y at will by pressing the proper lever. This experiment also teaches something about currents, but these will be discussed later.
=EXPERIMENT 104. To study the simple current detector.=
_Apparatus._ The compass (No. 18); dry cell, D C (No. 51); current reverser, C R (No. 57); copper wire, C W, a few feet long, with spring connectors on its ends. (See Apparatus Book,