Mr. L. Schwendler on Testing with Wheatstone's Diagram. 29 



G consists of three units, forms one-fourth of cube, and is the body ob- 

 tained by the third mode of dividing the cube. 



H. Cube composed of four of the above bodies, G. 



I. Four of the same bodies (G) reversed and rearranged to produce the half 

 of the rhomboidal dodecahedron. Another cube similarly divided and 

 arranged completes the solid. 



J. Six units (A) or three bodies (C) so arranged as to produce the rhom- 

 boidal cube — the basis of the hexagonal system. Seven of these bodies 

 build up the bee's cell. 



K. The cube divided by cutting off four three-sided pyramids, leaving a 

 tetrahedron in the centre. 



The four three- sided pyramids cut off may be so arranged as to pro- 

 duce the half of the true octahedron. 



L. Rhomboidal dodecahedron with pyramids (C) on each of the twelve faces. 

 This body contains forty-eight units (A). 



M. The remainder of the large cube, having an edge of two inches, consists 

 of forty-eight units (A) so arranged as to show how the rhomboidal 

 dodecahedron (L) can be inserted in the vacant space. 



V. On the Galvanometer Resistance to be employed in testing with 

 Wheatstone's Diagram. By Louis Schwendler, Electrician 

 to Siemens Brothers*. 



THE Philosophical Magazine of May 1866 contains an article 

 on the above subject, in which I showed that the maxi- 

 mum magnetic effect upon the needle is obtained when 



(a + d)(b + c) 



y- a+b+c+d' w 



g being the galvanometer resistance, and a, b, c, and d the four 

 branch resistances; and I have there already mentioned that the 

 above expression for g is only correct under the following three 

 conditions : — 



1. The resistance in the battery branch must be small, as 



compa r ed with the parallel resistance of the two double branches 



which a re opposite the galvanometer-branch, i. e. small in propor- 



,. . ( a + d){b + c) 



tion to ~ J- 



a + o + c-t-a 



2. Balance must be almost established, and 



3. The sectional area of the insulating covering must be small 

 in proportion to that of the conductor ; or, to express this more 

 generally, the proportion of the non-conducting to the conducting 

 sectional area must be a constant value for wires of various dia- 

 meters. 



The first and second conditions are fulfilled in all cases of 

 practical interest; but not so the third, as the thickness of silk 

 covering is always the same for thin as for thick wires. Thus a 



* Communicated by the Author. 



