_=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. [Illustration: Fig. 69.] =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. [Illustration: Fig. 70.] =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)?