OF ARTS AND SCIENCES. 433 



at the ends, like four battery cells connected for quantity. Between 

 the poles of this magnet a Siemens' armature is revolved, and both 

 magnet and armature are included in the main circuit. The instru- 

 ment is therefore extremely simple, and, when the circuit is broken, 

 requires no power to run it except to overcome the friction of the 

 bearings. The total weight is about 700 lbs., and the dimensions 33.5 

 by 21.5 inches (85 X 55 cms.), with a height of 14 inches (37 cms.). 

 To avoid heating, a water space is left close to the armature, but this 

 is required only when the resistance of the circuit is small. 



The quantities to be measured were as follows: 1st, velocity of 

 rotation of armature ; 2d, power required ; 3d, strength of current 

 with various speeds and resistances ; 4th, electro-motive force under 

 the same conditions ; 5th, when the current is used to produce a light, 

 a measure of the latter is candle-power. 



Power. The boilers and engine of the Institute were used as a 

 source of power. The nominal capacity of the boilers was sixty-six, 

 and of the engine fifteen horse-power; but, owing to various difficul- 

 ties beyond the control of the writers, only a small portion of this was 

 available, and that only for limited periods of time. A belt passed 

 from the fly-wheel of the engine over a countershaft in the Physical 

 Laboratory, giving it a velocity of about 500 turns per minute. A set 

 of five cone pulleys were attached, by which a speed of 333, 410, 500, 

 610, and 750 turns could, by shifting a belt, be given to a second 

 shaft. The latter carried a wheel 20 inches in diameter, and drove the 

 machine by a belt passing over a pulley 8 inches in diameter attached 

 to the armatui'e. As the speed of the engine varied somewhat, a 

 speed of from 800 to 2100 turns per minute was thus obtained. 

 Various plans were tried to measure the power emjiloyed. For the 

 earlier experiments a Batchelder dynamometer was used, in which the 

 motion was transmitted through four bevel-gears, and the moment of 

 tension measured by a spring-balance and weights. The instrument 

 was not however intended to be run at such high speeds, and the gears 

 were very noisy. 



Speed. The number of revolutions per minute is so important a 

 factor in these measurements that it must be constantly determined. 

 At first, a common shaft speeder was employed ; but, apart from its want 

 of accuracy, its constant use was laborious, and it showed only the total 

 number of turns during a minute, and not the speed at any interme- 

 diate instant. A device was accordingly employed, constructed by 

 Mr. J. B. Ilenck, Jr., by which these difficulties are completely 

 avoided. The plan is not new, having been published in a modified 

 VOL. X. (x.s. II.) 28 



