expressed in units, the equation becomes: maxwells = gilberts / oersteds _The gilbert is the unit of magnetomotive force, equivalent to the magnetomotive force of .7958 ampere turn._ It should be noted that in the electric circuit resistance causes heat to be generated and therefore energy to be wasted, but in the magnetic circuit reluctance does not involve any similar waste of energy. =Ques. Upon what does the reluctance of a magnetic circuit depend?= Ans. _The reluctance is directly proportional to the length of the circuit, and inversely proportional to its cross sectional area_. The reluctance of a magnetic circuit is calculated according to the following equation: reluctance = length in centimetres / (permeability × cross section in square centimetres) =Hysteresis.=--The term hysteresis has been given by Ewing to the subject of _lag of magnetic effects behind their causes_. Hysteresis means to “lag behind,” hence its application to denote the _lagging of magnetism, in a magnetic metal, behind the magnetizing flux which produces it_. =Ques. What is the cause of hysteresis?= Ans. It is due to the friction between the molecules of iron or other magnetic substance which requires an expenditure of energy to change their positions. [Illustration: FIGS. 124 and 125.--Experiment illustrating the molecular theory of magnetism. Coarse steel filings are placed inside a small glass tube and the contents magnetized. It will be found that filings which at first had no definite arrangement will rearrange themselves under the influence of magnetic force, and assume symmetrical positions, each one lying in line with, or parallel to its neighbor, as shown in the lower figure.] =Ques. When do the molecules change their positions?= Ans. Both in the process of magnetization and demagnetization. =Ques. What becomes of the loss of energy due to hysteresis?= Ans. It is converted into heat in changing the positions of the molecules during magnetization and demagnetization. Ewing gives the value for the energy in ergs dissipated per cubic centimetre, for a complete cycle of doubly reversed strong magnetization for a number of substances as follows: Substance Energy dissipated (ergs) Very soft annealed iron 9,300 Less “ “ “ 16,300 Hard drawn steel wire 60,000 Annealed “ “ 70,000 Same steel glass hard 76,000 Piano steel wire annealed 94,000 “ “ “ normal temper 116,000 “ “ “ glass hard 117,000