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Mr. C. W. Siemens on certain means of [Jan. 30, 



the drawing, the contact pieces are all separate, and the current has 

 to pass through the entire series of coils, which present sufficient 

 aggregate resistance to prevent the current from exceeding the desired 

 limit. 



When the minimum current is passing, the thin metallic strip is 

 at its minimum working temperature, and all the metallic prisms 

 •are in contact, this being the position of least resistance. As soon 

 as the current passing through the apparatus shall increase in 

 amount, the thin metallic strip will immediately rise in temperature, 

 which will cause it to elongate, and will allow the lever (L) to 

 recede from its extreme position, liberating one contact piece after 

 another. Each such liberation will call into action the resistance 

 coil connecting the spring ends, and an immediate corresponding 

 diminution of the current through increased resistance ; addi- 

 tional resistance will thus be thrown into the circuit, until an 

 equilibrium is established between the heating effect produced by the 

 current in the sensitive strip, and the diminution of heat by radiation 

 from the strip to surrounding objects. In order to obtain uniform 

 results, it is clearly necessary that the loss of heat by radiation should 

 be made independent of accidental causes, such as currents of air or 

 rapid variations of the external temperature, for which purpose the 

 strip is put under a glass shade, and the instrument itself should be 

 placed in a room where a tolerably uniform temperature of say 15° C. 

 is maintained. Under these circumstances, the rate of dissipation by 

 radiation and conduction (considering that we have to deal with low 

 degrees of heat) increases in arithmetical ratio with the temperature of 

 the strip ; the expansion of the strip, which affects the position of the 

 lever (L), is proportionate to the temperature which is itself propor- 

 tionate to the square of the current — a circumstance highly favour- 

 able to the sensitive action of the instrument. 



Suppose that the current intended to be passed through the instru- 

 ment is capable of maintaining the sensitive strip at a temperature of 

 say 60° C, and that a sudden increase of current takes place in con- 

 sequence either of an augmentation of the supply of electricity or of a 

 change in the extraneous resistance to be overcome, the result will be 

 an augmentation of temperature, which will continue until a new 

 equilibrium between the heat supplied and that lost by radiation is 

 effected. If the strip is made of metal of high conductivity, such as 

 copper or silver, and is rolled down to a thickness not exceeding 0'05 

 milim., its capacity for heat is exceedingly small, and its surface 

 being relatively very great, the new equilibrium between the supply 

 of heat and its loss by radiation is effected almost instantaneously. 

 But, with the increase of temperature, the position of the regulating 

 lever (L) is simultaneously affected, causing one or more contacts to 

 he liberated, and as many additional resistance coils to be thrown 



