The power delivered to an electric circuit is at any instant equal to the product of the current strength in amperes, and the electromotive force or potential difference in volts, and this power is expressed in watts. With ordinary continuous currents the electromotive force and current strength remain constant for a finite time, and this constant electroanotive force and current strength is indicated on a voltmeter and an a~m:eter suitably connected in the circuit. The product of the v~,lts and amperes so indicated is the power in watts and is al ;o constant. \V2tl~ altc':~ating currents the electromotive force and current strength are constantly changing in accordance with some periodic taw. it is still true that the power is at any instant equal to the product of the volts and amperes at that instant. Alternating current voltmeters and ammeters do not indicate instantaneous values, but the square root of the az,erage sqttarcs of these instantaneous values, which are called in practice the *effective values. The average power is equal to the average of the products of the instantaneous volts and amperes, and *In this article effective values of c. m. f. and current are designated by capital italics E and I, and instantaneous values by small italic a e and i.