500 PEOFESSOR W. E. AYRTON, MR. T. MATHER AND MR. F. E. SMITH : 



leading from them terminate at the brass connectors inside the cylinder. These 

 connectors are grooved, and into the grooves stout pieces of copper wire are soldered 

 and lead directly to the concentric cable. The junctions are shown in fig. 10, p. 482. 

 To take part of the weight of the cable attached to the small cylinders and thus 

 prevent the connections from being strained, two small curved arms project from 

 each suspended cylinder, and to these the cables are clamped. They may be seen just 

 above the fixed cylinders in fig. 2, Plate 7. Each small cylinder is suspended by three 

 phosphor-bronze wiresw, w, w, fig. 6, Plate 8, attached to a three-limbed spiderS ; to these 

 wires the cylinders are hooked by brass strips screwed to the interior of the cylinder 

 and bent at right angles at their lower ends ; the feet thus formed fit into recesses 

 cut in the marble. The effective length of the phosphor-bronze wires is adjustable, 

 and by such adjustment, together with an alteration in position of the heavy nuts on 

 the limbs of the spider, the cylinders are levelled. On the completion of the sus- 

 pended coils and their fittings, the mass of one suspended system was different to that 

 of the other by 2 grammes in a total of 5500 grammes ; equality was obtained by 

 loading one of the T-pieces. 



Above each suspended cylinder a commutator C (fig. 17) is supported by one arm of 

 the three-limbed spider. The concentric cables from the coils pass to this commutator, 

 and from the latter two bare copper wires, shown black, are taken to an ebonite block B. 

 A second ebonite piece B' is screwed to the main pillar of the balance, and between 

 B and B' 160 silver wires are suspended ; the diameter of a single wire is 1 mil (25/u.). 

 A long length of concentric cable completes the circuit to a multiple commutator and 

 plug board. By appropriately setting the commutator C, the current can be made to 

 circulate in the same or in opposite directions in the two helices, fig. 3, and by 

 suitable connections to the multiple commutator and to the commutator C the 

 insulation resistance between the helices can be measured. Wherever possible, the 

 non-concentric leads to and from the coils are kept very short and placed radially or 

 parallel to the axes of the cylinders ; also, the feed and return leads are placed as 

 close together as practicable ; the design thus ensures the minimum of force between 

 the current in the fixed coils and that in the commutator and leads to the suspended 

 .coils. The commutator C is a simple one of four copper quadrants with a turning 

 head of ebonite, carrying two contact pieces ; these latter are insulated from each 

 other and are attached to the ebonite head by hard springy copper ; they are split 

 midway to ensure uniform pressure on all the quadrants when the turning head is 

 correctly set. The commutator can reverse the current in one of the helices only, 

 but the concentric leads from both coils pass to the commutator block ; this is for 

 convenience in making the connections, and to obtain symmetry of distribution of the 

 current leads. By making one of the contact pieces (say Q, fig. 17, a) slightly longer 

 than the other, the commutator may be set in position suitable for making the 

 insulation test between the two helices. 



The 160 silver wires are divided into two portions, which are insulated from each 



