Januabv 9, 1903.] 



SCIENCE. 



51 



cessive sparking at the brushes. For 

 momentary overloads relatively large cur- 

 rents may pass through the coils without 

 injury to the insulation, since the tem- 

 perature effect is cumulative and requires 

 time for its operation. However for con- 

 tinuous periods of considerable length it is 

 usually unsafe to operate the motor much 

 above its rated output. 



Ordinarily in machine-driving the motor 

 is shunt-wound, and the current through 

 the field-coils is constant under all condi- 

 tions of load; but to obtain the best re- 

 sults with that class of machinery in which 

 the load is intermittent and subject to 

 sudden variations, the motor should be 

 compound-wound so as to increase the 

 torque without an excessive increase of 

 current in the armature. 



In many cases with individual motors, 

 owing to wide variations in power re- 

 quired, the average efficiency of the motor 

 may be very low, for this reason a care- 

 ful consideration of the conditions govern- 

 ing each case indicates that for ordinary 

 machine-driving, especially with small ma- 

 chines, short lengths of light shafting may 

 be frequently employed to good advantage, 

 and the various machines, arranged in 

 groups, may be driven from one motor. By 

 this method fewer motors are required, and 

 each may be so proportioned to the aver- 

 age load that it may run most of the time 

 at its maximum efficiency. 



When short lengths of shafting are em- 

 ployed the alignment of any section is very 

 little affected by local settling of beams or 

 columns, and since a relatively small 

 amount of power is transmitted by each 

 section, the shaft may be reduced in size, 

 thus decreasing the friction loss. More- 

 over, with this arrangement, as also with 

 the independent motor, the machinery may 

 often be placed to better advantage in order 

 to suit a given process of manufacture; 



shafts may be placed at any angle without 

 the usual complicated and often unsatis- 

 factory devices, and a setting-up room may 

 be provided in any suitable location as re- 

 quired, without carrying long lines of 

 shafting through space. This is an im- 

 portant consideration, for not only is the 

 running expense reduced thereby, but the 

 clear head-room thus obtained, free from 

 shafting, belts, ropes, pulleys and other 

 transmitting devices, can be more easily 

 utilized for hoists and cranes, which have 

 so largely come to be recognized as essential 

 to economical manufacture. 



In arranging such a system of power dis- 

 tribution the average power required to 

 drive is of as much importance as the 

 maximum, for in a properly arranged 

 group system the motor capacity need not 

 be the equivalent of the total maximum 

 power required to operate the several ma- 

 chines in the group, but may be taken at 

 some value less than the total, depending 

 upon the number of the machines and the 

 average period of operation. On the other 

 hand as already shown, the motor capacity 

 of independently driven machines must not 

 only equal the maximum power required 

 to drive the machine at full load, but it 

 must be capable of exerting a greatly in- 

 creased momentary torque. In any case 

 large units should be avoided, for the multi- 

 plication of machines driven from one 

 motor entails additional shafting, counter- 

 shafts and belting which may readily cause 

 the transmission losses to be greater, than 

 those obtained with engines and shafting 

 alone, besides friistrating some of the prin- 

 cipal objects of this method of transmission. 



As far as the efficiency of transmission 

 is concerned, it is doubtful whether, in a 

 large number of cases, motor-driving per 

 se is any more efficient than well-arranged 

 engines and shafting. 



As already pointed out, the principal 



