32 BULLETIN OF THE LABORATORIES 



ness in the laboratory unless current can be drawn, directly or 

 indirectly, from a power dynamo. And here it may indeed be- 

 come a genera] laboratory convenience. Those are especially for- 

 tunate whose laboratories are equipped with direct current in- 

 candescent lamps. Such can very easily have all the luxuries 

 that have been mentioned. Motors from 1-12 horse power up, 

 that run at a perfectly constant speed under varying load are 

 common trade articles ; the numerous special uses of the incan- 

 descent lamp are directly provided for ; arc lamps for projection 

 purposes, adapted to such circuits, have reached a high state of 

 perfection and from the known voltage it is easy for any elec- 

 trician to calculate just what kind of a coil of wire will produce 

 any required heating effect. The voltage of such light mains 

 is too high for many purposes, such e. g. , as chemical analysis, 

 and for use with most of our demonstration apparatus whose 

 coils have been constructed for the low voltages of primary bat- 

 teries. But it can be reduced conveniently in several ways. 

 First, by use of an extra resistance with it; this is not econom- 

 ical in the use of current, but is simple and easily controlled. 

 Second, by using a motor adapted to this voltage to run a dy- 

 namo wound to give the desired kind of current ; this involves 

 the use ot two expensive machines. Some expense and space 

 may be saved by combining them in one apparatus — the motor- 

 dynamo, or dynamo with a double winding on its armature. 

 Third, by running the currents used for lighting the building, or 

 for other moderately constant uses, through a few cells of storage 

 battery, using this for low voltage currents. This seems the best 

 way. It is equally applicable to the case of those laboratories 

 not connected with electric light mains that have their own dy- 

 namo running but only a part of the time. The storage battery 

 can then be available for many purposes when the dynamo is 

 idle. For instance, suppose the dynamo generates current at 

 no volts, instead of using iio volt lamps, we may use 90 volt 

 lamps running the current for them through about 10 cells of 

 storage battery. If we use an average say of 20 lamps 10 



