August 19, 1898.] 



SCIENCE. 



207 



turned inward and are connected, by sLort 

 pieces of rubber tubing, to the ' galvanome- 

 ter' G. The other two T'& have their side pro- 

 jections turned outward, and these connect 

 by long pieces of tubing with two large aspi- 

 rator bottles, not shown ; a, 6, R and x are 

 brass strips held under a screw at the inside 

 end and resting over rubber tubes which can 

 be compressed and closed by the screws ssss. 

 These screws are made by soldering a piece 

 ■of brass about 15 x 25 x 1 mm., with corners 

 rounded, into the slot of a common wood 

 screw . Under jB is a piece of tubing several 



feet long when self-induction is to be shown ; 

 otherwise a short piece is used, like those 

 under a, h and x, just long, enough to con- 

 nect the corresponding T's. C is a pint 

 aspirator bottle, with its small side tubulure 

 inserted into the side of the rubber tube. 

 The ' galvanometer ' is a T of peculiar form, 

 as shown, closed at its top by a rubber tube 

 and plug or pinch cock, as shown at D. It 

 is really an electrometer, as it shows differ- 

 ence of pressure (potential) instead of cur- 

 rent. In operation the board lies horizontal 

 on a table and the tube G is stood up per- 



pendicular so that it becomes a manometer. 

 To start the appartus fill the large bottle 

 connected with B and set it higher than the 

 board, tilt the A end of the board up ten or 

 fifteen degrees, place the A bottle lower 

 than the B one and open the stop cock 

 (key) K. The water will flow through the 

 tubes, driving out the air, and a little ma- 

 nipulation of D will bring the water in the 

 manometer to a convenient point. The 

 board is then to be placed horizontal 

 again. By compressing a and b about 

 equally resistance to flow is introduced into 

 these two arms of the bridge; then compress- 

 ing X will require a corresponding compres- 

 sion of R in order to balance the pressure 

 in G. It will be readily seen that the 

 qualitative effects can be thus shown, but 

 for quantitative relations it would be neces- 

 sary to replace the crude compressors by 

 graduated stop cocks. The capacity can be 

 regulated by closing K, raising the A bottle 

 and taking the cork out of C, allowing it to 

 fill more or less and then replacing the 

 cork ; the ' capacity ' is proportional to the 

 air left in C When C is full there is no 

 capacity in x. Self-induction is propor- 

 tional to the length and curvature of the 

 long tube, /7, under R. Self-induction and 

 capacity are shown by a momentai'y differ- 

 ence in level in the manometer upon open- 

 ing and closing K, the throw (balistic gal- 

 vanometer) being in the opposite direction 

 on closing K to that upon opening K. 

 Changing the levels of the A and B bottles 

 will show that the results are independent 

 of the direction or value of the pressure 

 (electro-motive force). The manometer 

 (galvanometer) may be made more and 

 more delicate by turning it down more and 

 more nearly horizonal, and by projecting 

 the image of G upon a screen its operation 

 may be shown to a large class. 



Individual ingenuity will devise many 

 variations and improvements in details and 

 experiments; the fancied merits of the ap- 



