POLAR EFFECTS OF ELECTRICAL CURRENT 209 



of this instrument indicates one-millionth of an ampere. 

 The currents which usually have to be measured vary from 

 one to a hundred micro-amperes. A single instrument 

 provided with appropriate shunts enables us to measure all 

 these intensities. 



Another point which should be borne in mind is the possi- 

 bility of induction-currents being generated in the measuring 

 instruments themselves, when the main current is suddenly 

 turned on and off. In order to guard against any disturbance 

 that might be caused in this way, it is necessary at the 

 moment of experimenting on the plant to put the voltmeter 

 and the micro-ammeter out of action for the time. On the 

 completion of each experiment the e.m.f. and the current 

 employed are measured by means of the voltmeter and the 

 micro-ammeter. 



In experiments where still higher voltage may be 

 required this potential slide cannot be used, as the wires 

 become unduly heated. We may in such a case modify 

 the potential slide by replacing the platinoid wire with two 

 parallel troughs containing copper-sulphate solution. In 

 place of the spring-contacts two copper plates are here 

 employed, dipping into the solution. These, forming the 

 moving contacts e and e', are attached to the slide. With 

 this form of electrolytic potential slide a graduated e.m.f. 

 up to 100 volts may be easily obtained. The terminals a 

 and b are connected with a battery of the required number 

 of storage-cells from the installation in the laboratory. 



A more convenient arrangement is a portable set of 

 storage-cells, with a Potential Keyboard, by means of 

 which any voltage up to 100 volts may be readily obtained 

 by steps of 2 volts at a time. Small storage-cells of flat 

 form may be obtained 6 cm. broad, 2*5 cm. in depth, and 

 6 cm. in height. Fifty of these cells, giving a total e.m.f. 

 of 100 volts, may be packed in a small box 25 by 30 cm., 

 having a height of 7-5 cm. Ten of these cells are arranged 

 in units, and forty in four groups of ten each. 



As regards the former, the ten cells are arranged in 



