910 



SCIENCE 



[N. S. Vol. XLI. No. 1068 



the elementary course to the work of the ad- 

 vanced course. When expectation or require- 

 ment concerning quality of work advances in 

 correspondence with the advanced character 

 of the courses, justice is most nearly assured 

 by assigning approximately the same percent- 

 ages of grades A, B, C in all courses. 



The system as adopted emphasizes also the 

 idea that the proposed percentages may not be 

 precisely observed in any single class in a 

 single year, especially among the smaller 

 classes. But it is expected that the deficiencies 

 in the assignments of particular grades of one 

 year, will be balanced by the excesses of an- 

 other year, so that there will be no constant 

 tendency on the part of any instructor's grades 

 to deviate widely from the percentages agreed 

 upon. In very small classes the grades of a 

 single year may deviate more widely from the 

 ideal than those of the larger classes, but the 

 combined reports of several years are expected 

 to show essential approximation to the defini- 

 tions. 



Wm. E. Kellicott 

 GoucHER College 



SPECIAL ARTICLES 

 A SAFE PORTABLE LAMP BATTERY 



The use for class work in physiological 

 laboratories of zinc and ammonium chloride, 

 or other forms of cells, is inconvenient and 

 involves constant renewal. To supply large 

 classes with dry batteries becomes an item of 

 considerable expense. When the 110-volt di- 

 rect current is available " lamp batteries " (or, 

 properly speaking, lamp resistances) are more 

 convenient, and cheapen to use; and if they 

 are permanently installed under the work 

 bench where the student can not alter the con- 

 nections nor easily short circuit them, they are 

 safe enough. 



For many purposes, however, it is more 

 convenient to have the lamps mounted on a 

 piece of board six or eight inches square, so 

 that the battery can be carried anywhere about 

 the laboratory and connected with any socket 

 by means of a cord and plug. The great dis- 

 advantage of such a portable battery is that 

 with inexperienced students it may easily re- 



sult in a serious blow-out. Thus with the 

 ordinary arrangement of the lamps, as shown 

 in Fig. 1, if B is the live wire and A is the 

 grounded wire of the city lines (and one is 

 usually grounded), no harm results if B hap- 

 pens to come in contact with a gas or water 

 pipe. But if A is the live wire, and one hap- 

 pens (as there is an even chance of doing) to 

 have pushed the plug into the socket so that 

 the lamp is nearest to the grounded line, 

 then the whole pressure of the city system bears 

 upon any chance contact of F with any metal 

 object leading to ground. 



To avoid this danger the form of battery 

 shown in Fig. 2 has proved convenient. To 

 give the same current the lamps G and C in 

 Fig. 2 must be twice the size (twice the cur- 

 rent consumption and illuminating power, or 

 in other words half the resistance) of C in 



Fig. 1. As both sides of the battery are then 

 the same, it does not matter whether 4 or 5 is 

 the live wire, nor which way the plug is put 

 into the socket, li E ot P happens to touch 

 a grounded object, the lamp on that side merely 



