Sept. 28, 1882] 



NATURE 



527 



The electromagnet, A (Fig. 1), is fixed to a long and 

 heavy rack, c, which falls by its own weight and by the 

 weight of the electromagnet and the carbon fixed to it. 

 The length of the rack is equal to the length of the two 

 carbons. The fall of the rack is controlled by a friction 

 break, b (Fig. 3), which acts upon the last of a train of 

 three wheels put in motion by the above weight. The 



Fin. 3 .< 



break, B, is fixed a f . one end of a lever, b A, the othe: end 

 arrying a soft iron armature, F, easily adjusted by three 

 screws. This armature is attracted by the electromagnet, 

 E E (whose resistance is 1200 ohms), whenever a current 

 circulates through it. The length of the play is regulated 

 by the screw, V. The spring, L, applies tension to the 

 break. 



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Fig. 4 



The Regulator. — This consists of a balance and a 

 cut-off. 



The Balance (Figs. 4 and 5) is made with two sole- 

 noids, S and s', whose relative resistances is adjustible. 

 S conveys the main current, and is wound with thick wire 

 having practically no resistance, and S' is traversed by a 

 shunt current, and is wound with fine wire having a re- 

 sistance of 600 ohms. In the axes of these two coils a 



small and light iron tube (2 m.m. diameter and 60 m.m. 

 length) freely moves in a vertical line between two guides. 

 When magnetised it has one pole in the middle and the 

 other at each end. The upward motion is crontrolled by 

 the spring n t. The spring rests upon the screw, h, with 

 which it makes contact by platinum electrodes. This 

 contact is broken whenever the little iron rod strikes the 

 spring, n T. 



The positive lead from the dynamo is attached to the 

 terminal, b, then passes through the coil, s, to the ter- 

 minal, b', whence it proceeds to the lamp. The negative 



lead is attached to terminal a, passing directly to the 

 other terminal, a', and thence to the lamp. 



The shunt which passes through the fine coil, s', com- 

 mences at the point, P. The other end is fixed to the 

 screw, H, whence it has two paths, the one offering no 

 resistance through the spring, t n, to the upper negative 

 terminal, a' ; the other through the terminal, .1, to the 

 electromagnet of the break, M, and thence to the negative 

 terminal of the lamp, l'. 



The Cut-off. — The last part of the apparatus (Fig. 4) to 

 be described is the cut-off, which is used when there are 



