494 PROCEEDINGS OF THE AMERICAN ACADEMY 



Cleveland. — The armature made up of two shuttle-wound armatures 

 mounted on the same shaft but placed at right angles to each other. 

 This armature revolves in a strong field produced by two field magnets, 

 wound so as to have consequent poles at their middle. Instead of 

 brushes, four anti-friction rollers are used in connection with a four- part 

 commutator. The pressure of these rollers is adjusted by springs. 



Model Edison Dynamo. — A miniature machine of the Edison pat- 

 tern, but series- wound instead of shunt- wound. 



Diehl. — The field magnets hinged at the yoke so that the distance 

 of the pole pieces from the armature is capable of adjustment, and the 

 armature made up of two Siemens shuttle-wound armatures at right 

 angles to each other on the same shaft. The motor is shunt-wound. 

 Two sizes were tested. 



Hill. — The field magnets are placed below the armature, which re- 

 volves between cast-iron pole pieces. The armature consists of a 

 series of eight spools of wire arranged in couples, the axes of adjacent 

 couples being at right angles to each other. The motor is series- 

 wound. Two sizes were tested. 



The efficiency of a motor is equal to the horse-power delivered, 

 divided by the rate of consumption of electrical energy by the motor ; 

 that is, if H. P. = horse-power delivered, C = current in amperes, and 

 E = electro-motive force in volts as measured at the terminals of the 



machine, efficiency = q ^ ' 



746 



The instruments used in the determination of C and E were Sir 

 William Thomson's current and potential galvanometers. These 

 had previously been carefully calibrated and found to be coi-rect to 

 within 0.2% in the positions at which they were used in the tests. 

 In cases where the compensating magnets had to be used, their in- 

 tensity was carefully determined on each day of use, so that no material 

 error should be introduced from this source. As far as possible, how- 

 ever, the use of the magnets was avoided. 



For measuring the power given out by the smaller motors a raw- 

 hide belt or a cotton cord was passed completely around a brass pulley 

 on the motor shaft, the upper end being attached to a spring balance 

 and the lower to a scale pan. By varying the weight in the scale pan 

 the speed of the motor could be changed. A Chatillon balance weigh- 

 ing to ^ oz. was used. In the tests on the Gramme Magneto and the 

 Gramme " j\ petite lumiere," the machines were placed on a cradle 

 dynamometer, a modification of the form devised by Prof. Brackett of 

 Princeton, and a dynamo machine was used as a friction brake. By 



