522 PROFESSOR W. E. AYRTON, MR. T. MATHER AND MR. F. E. SMITH: 



plug board divided into two sections for the left-hand and right-hand sets of coils 

 respectively. A commutator allows of the reversal of the current in all the coils on 

 any one cylinder, and the plug board allows of the reversal of the current in any one 

 or more helices on the fixed cylinders. The inner and outer conductors of the 

 concentric cables, D, fig. 25, are soldered to thin brass tubes let into brass blocks and 

 pass to the various coils of the balance. Each helix is designated by a word, a letter, 

 and a number, which are marked on an ebonite bridge at the top of fig. 25 ; the 

 turning heads are also marked so as to enable reversals of the current to be quickly 

 made without likelihood of error. Each plug hole is numbered, and a scheme was 

 drawn up so that any desired combination was represented by a series of numbers 

 for the plugs and by letters for the commutators. For example, in taking a (D + S) 

 observation, plugs are inserted in the holes 13, 2, 3, 16, 5, 18, 19, 8, 21, 10, 11, 24, 

 and the four commutators arranged in positions represented by the diagram a, fig. 25. 

 Here the straight lines in the circle represent the directions of the turning heads 

 of the commutators. When the main commutator M, fig. 23, is in the A position, 

 and the multiple commutators as shown in a, fig. 25, the whole arrangement is 

 designated by the symbols A. Reversing the main commutator changes it to aB. 

 Turning the commutators connected with the suspended cylinders to the positions 

 indicated by diagram b, fig. 25, we get an arrangement symbolised by 6B, and a 

 reversal of the main commutator gives 6A. Changing from A to 6B reverses 

 the current in the fixed coils only, as also does the change 6B to e*A. In fig. 25 the 

 letters DD, &c., indicate the ten concentric cables running from the plug board to 

 the balance. 



The reversal of the current in one of the two helices on a suspended cylinder is 

 made by a small commutator on the three-limbed spider. This has been described 

 on p. 500, and is illustrated in fig. 17. 



Balancing Masses. The weights employed are eight in number and are made of 

 aluminium. They are divided into two sets: four for the (D + S) observations and 

 four for the (D S) observations, and the masses of the weights in each set are 

 approximately equal. The force due to one (D + S) weight very nearly balances half 

 the total force due to the current in such observations, and it may be employed for 

 observations of the maximum force when the current flows through all the coils of 

 one system and through the suspended coils only of the other system. Similarly, a 

 (D S) weight may be used for observations of the minimum force under such 

 conditions. For the (D + S) observations two weights are used on each side of the 

 balance, similarly for the (D S) observations, and the total mass of the eight weights 

 is required for the calculationa The masses of different combinations were, however, 

 also determined. The standard mass employed was a 100-gramme weight standardised 

 at Sevres, and the effective mass of the eight weights in a medium of density O'OOl 196 

 was determined as 31 '12494 grammes, the four (D + S) weights being 1573135 

 grammes and the four (D S) weights being 15 '39359 grammes. Aluminium is not 



