_=308. Unit of Resistance; The Ohm.=_ Whenever anything is to be measured, a standard, or unit, is necessary. The unit of resistance is called the ohm, in honor of Ohm, who made careful investigations upon this subject. A column of mercury having a length of a little over 3 feet has been taken as a unit. (The column taken is 106.3 cm. with a weight of 14.4521 grams; it has a cross-section of about 1 sq. mm., at a temperature of 0°C.) Mercury is a liquid, and has no "grain" to affect the resistance. For the use of students, 9 ft. 9 in. of No. 30 copper wire, or 39 ft. 1 in. of No. 24 copper wire will make a fairly good ohm. We might, of course, take any other length as _our_ standard; the above, however, will give results that are approximately correct. (See wire tables at the end of this book.) _=309. Resistance Coils; Resistance Boxes.=_ Coils of wire, having carefully-measured resistances, are called _resistance coils_. The wire for any coil is doubled at the center before it is wound into coils or upon spools (Fig. 93) to avoid the magnetic effect. The ends of the coils are attached to binding-posts, or to brass blocks, in regular instruments, so that one or more coils can be used at a time; that is, so that they may be handled in a manner similar to that in which the different coils on the galvanoscope are used. If we have 4 coils of 1, 2, 2, and 5 ohms resistance, we shall be able to use any number of ohms from 1 to 10 by making the proper connections. (See Apparatus Book, chapter XVII, for Home-made Resistance Coils.) For protection and convenience, coils are usually placed in a box, the whole being called a _resistance box_. The ends of the coils are joined to brass blocks, placed near each other on the top of the box, and between which may be pressed plugs when it is desired to short circuit the coils. By removing a plug, the coil, whose ends are joined to the blocks touching it, is brought into the circuit. [Illustration: Fig. 93.] [Illustration: Fig. 94.] =310. Simple Resistance Coil.= Fig. 94 shows a simple form of coil, R C (No. 79). The total resistance is 2 ohms, L (left) and R (right) being binding-posts to which the ends of the coil, C, are joined. M (middle) connects with the middle of the wire, at which point the wire is doubled. The coil is fastened to a stiff pasteboard base, B. =Connections.= When 2 ohms resistance are wanted, let the current enter at L and leave at R (or the reverse). When 1 ohm is wanted, let the current leave or enter at M, the other wire being joined to L or to R. Connections should be made with spring connectors. See § 229. =EXPERIMENT 119. To test the power of various substances to conduct galvanic electricity.=