April 26, 1889.] 



SCIENCE. 



315 



secohmmeter, and increase or diminish one of the resistances, r-^, 

 for example, by a small amount p, obtaining a steady deflection, 

 d.;^, of the galvanometer with the battery previously used ; then 

 d^ r^ p 

 L = — . — . — secohms approximately. 

 d-i r^ 8« 

 For this test the relative positions of the two commutators is un- 

 important. They may be as in Figs. 2 and 3, in which case the 



FIG. 3. — COMPARING TWO CAPACITIES. 



reversal of the galvanometer occurs midway between two consecu- 

 tive reversals of the battery ; or they may be as in Fig. 4, in which 

 case the reversal of the galvanometer just precedes the reversal of 

 the battery. The greater the value of n, the greater will be the 

 deflection d^, and the more accurately can it be read ; but the 

 speed must not be too great to prevent the currents reaching their 

 steady values between two consecutive reversals of the battery. 

 Whether this condition be fulfilled or not, can be best ascertained 



FIG. 4.— CO-EFFICIENT OF SELF-INDUCTION IN SECOHMS. 



by seeing whether the same value is obtained for L, for a speed 

 considerably smaller than n. 



The sensitive zero method is as follows : Instead of obtaining 

 two deflections, the resistances, r^, r^, r^, are first adjusted to 

 give balance with a steady current, and then one of them, say r^, 

 is altered by an amount c ohms, so that there is still no deflection 

 of the galvanometer when the commutator spindle makes, say, n. 

 revolutions per second ; then 



rj k 

 Z = — a - secohms approximately, 



^2 n 

 where /4 is a constant depending on the relative position of the 



commutators. The value of the constant k is most accurately as- 

 certained once for all, for a given relative position of the commu- 



tators, by experimentally determining the value of — a, that pro- 

 p's 

 duces balance for a known co-efficient of self-induction, when the 

 commutator spindle is driven at some known number of revolutions 

 per second. 



For this latter test it is necessary that the commutators be so 

 placed relatively to one another that the galvanometer is not re- 

 versed exactly midway between two consecutive reversals of the 

 battery ; since, with this latter adjustment, no variation in the re- 

 sistance of any of the arms of the bridge can counterbalance the 

 effect of the self-induction on rotating the secohmmeter handle : in 

 fact, the more nearly the commutators are placed in the midway 

 adjustment, the smaller will be the value of k, and therefore the 



larger the value of — a, to produce balance for given values of I^ 



and 71. 



5. To measure the resistance of a polarizable electrolyte, replace 

 the coil having self-induction in Fig. 4 by the polarizable electro- 

 lyte ; adjust the commutators so that the galvanometer is reversed 

 just before the battery ; and, using the higher speed ratio for the 

 gearing, rotate the secohmmeter at the highest convenient speed. 

 Then, if x be the true resistance of the electrolyte, 



THE RATTLESNAKE'S RATTLE. 

 Mr. S. Garman of the Museum of Comparative Zoology, Cam- 

 bridge, Mass., has been investigating the rattle of the rattlesnake. The 

 habit of sloughing is common to all serpents. A short time before 

 the removal of the old skin takes place, the new epiderm makes its- 

 appearance beneath the old. The mode of growth of the new and' 

 the removal of the old is the same in all snakes, with the excep- 

 tion that in those with a rattle that portion of the slough that 

 covers the tip of the tail is retained to form one of the rings of the- 

 rattle. The attachment is simply mechanical : the rings are merely 

 the sloughs off the end of the tail. The terminal bone of the tail 

 is formed of vertebrae that have coalesced, and changed in great 

 measure their shape. In the different species the number of ver- 

 tebras included in this bone varies considerably, and sometimes it 

 varies in individuals of the same species. With the purpose of in- 

 dicating the manner of growth of the rattle, and as far as possible 

 determining its origin, Mr. Garm an has followed up its appearance 

 in several species, full details of which, with figures, have been 

 lately published. In the very young rattlesnake, while the verte- 

 bra are still separate, there is no rattle ; but about a week after 

 birth a well-marked button is seen. With the first slough the first 

 ring is set free, the button being pushed forward, and a third but- 

 ton is gradually perfected. In time the traces of the vertebrse in 

 the terminal bone are almost obliterated. The bone becomea 

 thickened, pushed forward at its edges, and otherwise enlarged. In-> 

 a full-grown rattlesnake the hinder seven of the rings belong to the 

 period of the snake's most rapid growth, — they form the " taper- 

 ing rattle " formerly used in classification of the species, — while- 

 four of the rings and the button are formed while the gain in size 

 was less rapid, and form the " parallelogrammic rattle " of the old 

 classifiers. Many serpents besides those possessed of a " crepitac- 

 ulum " are addicted to making a rattling noise byjvibrations of 

 the end of their tails. In illustration of the extent to which the 

 tail has been modified in different cases, Mr. Garman figures the 

 tails of several species, among others that of Ancisirodott con/or— 

 trix, Lin., the copperhead of the United States. The tip of its tail/ 

 is directed downwards as well as a little backwards. Most often; 

 the button has one or two swellings in a degree resembling those 

 on a ring of the rattle. A living specimen of this snake, kept for a 

 year or more, would take to rattling on the floor whenever it was 

 irritated. The sound was made by the terminal inch of the tail, 

 this part being swung from side to side in the segment of a circle, 

 so that the tip might strike downward. The result was a tolerable 

 imitation of the sound made by a small rattlesnake.. 



