|§ BULLETIN OF THE LABORATORIES 



justment of the instrument. If the resistances of the various pieces of 

 electrical apparatus in the laboratory are measured, a moment's calcula- 

 tion will show just what arrangement will give the greatest amount of 

 current, or any amount desired, with a certainty and precision that is very 

 satisfactory. This increased precision is possible with so little trouble- 

 by the use of this cell-connecter that it is well worth one's while to 

 gain it. 



Larger batteries can be managed either with a similar instrument 

 having a greater number of i)lates and connecting plugs, or by attach- 

 ing to each pair of plates in the form described, a battery of cells, ar- 

 ranged in series or parallel, according as high potentials or heavy cur- 

 rents are most recpiired. 



Another form of the instrument has been tried. It is more easily 

 made, but is less convenient in use. It consists simply of two rows 

 of binding posts on a base. These are connected by short wires in 

 various ways, for various combinations Two rows of metal contact 

 points, each carrying a sliding switch which could be placed upon the 

 ne.xt point of either the same or the other row, would also be a conve- 

 nient form. By this method, however, there would be difficulty in 

 securing good contact betw^een points and switches. 



This instrument is especially useful for a complete experimental 

 proof of Ohm's law. A student with one of them, an ammeter and 

 voltmeter, or in place of the last two. a tangent galvanometer having 

 both low and high resistance coils, can verify the law for a large num- 

 ber of representative combinations in a wonderfully short time. 



It has been found a great convenience in this laboratory to have 

 one of them upon the demonstrator's desk, with a dozen open-circuit- 

 cells attached. These require no attention for six months at a time, 

 are always ready for use and do not polarize seriously in the short time 

 necessary for an ordinary lecture experiment. The "old reliable" of 

 the laboratory, a ten-cell plunge battery, has been thrown out of em- 

 ployment since the advent of the cell-connecter. It is surprising how 

 much can be done by the open circuit cells, with the aid of their pow- 

 erful auxiliary. 



It is hoi)e(l that it may i)rove as great a convenience in other lab- 

 oratories as it has in this. Evidently it can be used with secondary 

 batteries as well as primary. In this way it gives perfect control of 

 very heavy currents. 



