3. The resistance is equal to the E. M. F. (electro-motive force) divided by the current strength. All these are different forms of the same statement; and when figuring electrical data, C stands for current, E for electro-motive force, and R for resistance. Resistance-coils and Rheostats The method by which electricity is controlled is resistance. No matter how great the voltage of a current, nor its volume in amperes, it can be brought down from the deadly force of the electric trolley-current to the mild degree needed to run a small fan-motor, an electric bell, or a miniature lamp. This is accomplished by means of resisting mediums, such as fluids or wires, which hold back the current, and allow only the small quantity to pass that may be required to operate the apparatus. The jump from the high voltage of the trolley-current to the low one required for the electric bell, a lamp, or a small motor, is frequently made in traction-work, but in this case regular transformers are used. For the small apparatus, that may have its current supplied from a battery, or a small dynamo driven by a water-motor, the forms of resistance-coils and rheostats described on the following pages should meet every requirement. The standard unit of resistance is called an ohm, so named after Dr. G. S. Ohm, a German electrician, whose theory on the subject is accepted as the basis on which to calculate all electrical resistance. The legal ohm is the resistance of a mercury column one square millimetre in cross-sectional area and one hundred and six centimetres in length, and at a temperature of 0° Centigrade or 32° Fahrenheit, or the freezing-point for water. The conductivity of metals is dependent greatly on their temperature, a hot wire being a much better conductor than a cold one. Since counter-electro-motive force sometimes gives a spurious resistance, the ohmic resistance is the true standard by which all current is gauged. In technical mechanism and close readings the ohmic resistance counts for a great deal, but in the simple apparatus that a boy can make the German-silver resistance coils and the liquid resistance will answer every purpose. To give a clearer idea of the principle of the rheostats, a short description of the mercurial column will first be presented. During the early part of the last century wires were not used as a resisting medium for electric currents. In their place, glass tubes, filled with mercury sealed at one end and corked at the other, were arranged in rows and supported in a wooden rack. [Illustration: _=Fig. 1=_]