THE SMITHSONIAN INSTITUTION. 409 



On the movable wooden cylinder are placed four semicircular wooden 

 bars, 90° apart, which are partly covered with bands of copper ; the 

 springs of the high jjosts rub upon these alternately dtiring the revo- 

 lution. 



On the bar which is represented as uppermost in the cut, and on 

 which the springs are resting, the copper bands are so arranged that 

 1 is brought into conducting connexion with 0, 2 with 3, 4 with 5, 6 

 with 7, and 8 with H ; in like manner the platinum plates from 1 to 8 

 are combined according to the principle of the battery, and closed by 

 the voltameter. 



The lower wooden bar has exactly the same construction as the 

 upper one. 



The other two bars opposite each other, to the right and left of 

 the cylinder, are also alike, and so constructed that when they come 

 into contact with the springs, the plates 2, 4, 6, and 8 are in conduct- 

 ing connexion with the carbon cylinder, and 1, 3, 5, and 7 with the 

 zinc cylinder of the charging element. 



For ready expression, we shall call tlie rollers which are above and 

 below in the cut the discharging rollers ; the others the charging 

 rollers. 



The construction of the charging rollers is as follows : Eight cop- 

 per bands are placed on the wooden roller in such a way tliat they 

 may come in contact with the eight springs corresponding to the eight 

 jjlatinum plates. Half of these bands (the 2d, 4th, 6th, and 8th 

 from the first in one figure) are connected with a copper strip, which 

 passes to the front ring of the cylinder, and thus to P. In the same 

 manner the other half of the copper bands (1, 3, 5, and 7) are con- 

 nected with a similar strip of copper, which, lying on the other side 

 of the wooden bar, is not visible in the figure, and which passes to 

 the farther copper ring of the cylinder, thence to Z ; thus the bands 

 1, 3, 5, and 7 are in connexion with the zinc cylinder, and 2, 4, 6, 

 and 8 with the carbon cylinder of the charging element, when the 

 charging roller is uppermost. 



The cylinder is turned by the crank ; at each revolution there is a 

 double charge and discharge of the secondary pile. The most suita- 

 ble dimensions for the cylinder are 12 centimetres long, (for a pile of 

 4 pair of plates,) and (without the bars) 2| to 3 centimetres in diam- 

 eter. 



It is well known that with one Grrove's element very little water can 

 be decomposed ; the voltameter plates become coated with gas bubbles, 

 but very i'ew ascend. But on using the simple battery through the 

 medium of the pole-changer for charging the secondary battery, in 

 whose circuit the voltameter is inserted, a lively decomposition of 

 water is obtained as soon as the pole-changer is set in motion, which 

 is a striking proof that the electro-motive force of the secondary cur- 

 rent is considerably stronger than that of the primary. 



With a voltameter whose plates presented a surface on each side of 

 about 3 square inches, sulphuric acid being added to the water, Pog- 

 gendorflf obtained from 5 to 6 cubic centimetres of explosive gas per 

 minute, when in this time the circuit was closed and opened about 80 

 times. 



