532 The Rev. T. R. Robinson's Experimental Researches on the 



transmitted to it by induction, two through the shorter distances between it and 

 the centres of the helices, the other two through the complements of the cir- 

 cuit. When any of these distances is increased, the corresponding force dimi- 

 nishes. The eifect of lengthening the entire circuit is considerably greater than 

 that of lengthening the distance of the helices from the keeper. 



The resultants of pairs of these forces can be obtained separately, but they 

 cannot be further separated, as the law of this magnetic composition is not yet 

 known. 



The law of the inductive transmission of the magnetic force is equally un- 

 known. It is not impeded by the magnetism of iron. 



Whatever impedes this transmission lessens the power of the magnet. 



The interposition of brass plates 0'*12 thick reduces it to a quantity vary- 

 ing from 0"24: to 0'03 of its normal amount, according to the nature of the 

 magnet. In this case the force is as -f ^ and, therefore, the development of 

 magnetism as •^. 



Even the interval between the keeper and polar surfaces, though the con- 

 tact be as close as possible, reduces it to 0'91, or 0'95. 



Complete interruption of the circuit in the middle of the keeper reduces it 

 to a value between 0"09 and 0"03, and one in the centre of the magnet from 

 0"70 to 018, the diminution being less as the- circuit is longer, and the spires 

 more uniformly distributed over it. 



When both interruptions exist, the two halves of the magnet act as sepa- 

 rate bar magnets ; but in this case the power, when greatest, is only a thirtieth 

 of the normal value. 



A magnet, intended to act with the circuit closed, should have its spires col- 

 lected as near the keeper as possible, and be as short as is compatible with 

 having the necessary quantity of wire coiled on it ; if it be designed to act at 

 a distance, it should be of considerable length, and have the spires uniformly 

 distributed over it. 



If the resistance of one of the helices be greater than the internal resistance 

 of the battery used, power is gained by employing them collaterally, but the 

 expenditure of materials is doubled. 



In magnets of the same length, but different diameters, the actions of similar 

 spires are as these diameters. 



