Part 4 (1/2)
[Ill.u.s.tration: FIG. 19.--Diagram of wiring of differential circuit with its various shunts, used in connection with resistance thermometers on water-circuit of bed calorimeter.]
Provision is made for automatically moving the contact _q_ by electrical means and thus the complete balance of the two differential circuits is maintained constant from second to second. As the contact _q_ is moved, it carries with it a stylographic pen which travels in a straight line over a regularly moving roll of coordinate paper, thus producing a permanently recorded curve indicating the temperature differences. The slide-wire J is calibrated so that any inequalities in the temperature coefficient of the thermometer wires are equalized and also so that any unit-length on the slide-wire taken at any point along the temperature scale represents a resistance equal to the resistance change in the thermometer for that particular change in temperature. With the varying conditions to be met with in this apparatus, it is necessary that varying values should be a.s.signed at times to J and to _r_. This necessitates the use of shunts, and the recording range of the instrument can be easily varied by simple shunting, _i. e._, by changing the resistance value of J and _r_, providing these resistances unshunted have a value which takes care of the highest obtained temperature variations.
Fig. 19 shows the differential circuit complete with all its shunts. S is a fixed shunt to obtain a range on J; S' is a variable shunt to permit very slight variations of J within the range to correct errors due to changing of the initial temperatures of the thermometers; _y_ is a permanent shunt across the galvanometer coil _fl_, to make the temperature coefficients of _fl_ and _fr_ absolutely equal; Z is the variable resistance in the battery-circuit to keep the current constant; _r_ is a permanent resistance to fix the zero on varying ranges; S''
plus S_{1} const.i.tutes a variable shunt to permit slight variations of _r_ to finally adjust 0 after S' is fixed and _t_ is a permanent shunt across the thermometer T_{1} to make the temperature coefficient of T_{1} equal to that of T_{2}.
The apparatus can be used for measuring temperature differences from 0 to 4 or from 0 to 8. When on the 0 to 8 range, the shunt S is open-circuited and the shunt S' alone used. The value of S, then, is predetermined so as to affect the value of the wire J and thus halve its influence in maintaining the balance. Similarly, when the lower range, _i. e._, from 0 to 4, is used, the resistance _r_ is employed, and when the higher range is used another value to _r_ must be given by using a plug resistance-box, in the use of which the resistance _r_ is doubled.
The resistance S'' and S_{1} are combined in a slide-wire resistance-box and are used to change the value of the whole apparatus when there are marked changes in the position of the thermometric scale. Thus, if the ingoing water is at 2 C. and the outcoming water at 5 C. in one instance, and in another instance the ingoing water is 13 and the outgoing water is 15, a slight alteration in the value of S_{1}, and also of S', is necessary in order to have the apparatus draw a curve to represent truly the temperature differences. These slight alterations are determined beforehand by careful tests and the exact value of the resistances in S' and in S_{1} are permanently recorded for subsequent use.
THE GALVANOMETER.
The galvanometer is of the Deprez-d'Arsonval type and has a particularly powerful magnetic field, in which a double coil swings suspended similar to the marine galvanometer coils. This coil is protected from vibrations by an anti-vibration tube A, fig. 20, and carries a pointer P which acts to select the direction of movement of the recording apparatus, the movable contact point _q_, fig. 19. In front of this galvanometer coil and inclosed in the same air-tight metal case is the plunger contact Pl, fig. 21. The galvanometer pointer P swings freely below the silver contacts S_{1} and S_{2}, just clearing the ivory insulator _i_. The magnet plunger makes a contact depending upon the adjustment of a clock at intervals of 2 seconds. So long as both galvanometer coils are influenced by exactly the same strength of current, the pointer will stand in line with and immediately below _i_ and no current pa.s.ses through the recording apparatus. Any disturbance of the electrical equilibrium causes the pointer P to swing either toward S_{1} or S_{2}, thus completing the circuit at either the right hand or the left hand, at intervals of 2 seconds. The movement of the pointer away from its normal position exactly beneath _i_ to either S_{1} on the left hand or S_{2} on the right, results from an inequality in the current flowing through the two coils in the galvanometer. The difference in the two currents pa.s.sing through these coils is caused by a change in temperatures of the two thermometers in the water circuit.
[Ill.u.s.tration: FIG. 20.--Diagram of galvanometer coil used in connection with recording apparatus for resistance thermometers in the water-circuit of bed calorimeter. A, anti-vibration tube; P, pointer.]
THE CREEPER.
The movement of the sliding-contact _q_, fig. 19, along the slide-wire J, is produced by means of a special device called a creeper, consisting of a piece of bra.s.s carefully fitted to a threaded steel rod some 30 centimeters long. The movement of this bar along this threaded rod accomplishes two things. The bar is in contact with the slide-wire J and therefore varies the position of the point _q_ and it also carries with it a stylographic pen. The movements of this bar to the right or the left are produced by an auxiliary electric current, the contact of which is made by a plunger-plate forcing the pointer P against either S_{1} or S_{2}. P makes the contact between Pl and either S_{1} or S_{2} and sends a current through solenoids at either the right or the left of the creeper. At intervals of every 2 seconds the plunger rises and forces the pointer P against either S_{1}, _i_, or S_{2} above. The movement of this plunger is controlled by a current from a 110-volt circuit, the connections of which are shown in fig. 22. If the contact is made at T, the current pa.s.ses through 2,600 ohms, directly across the 110-volt circuit, and consequently there is no effective current flowing through the plunger Pl. When the contact T is open, the current flows through the plunger in series with 2,600 ohms resistance. T is opened automatically at intervals of 2 seconds by the clock.
[Ill.u.s.tration: FIG. 21.--Diagram of wiring of circuits actuating plunger and creeper.]
[Ill.u.s.tration: FIG. 22.--Diagram of wiring of complete 110-volt circuit.]
The movement of the contact arm along the threaded rod is produced by the action of either one of two solenoids, each of which has a core attached to a rack and pinion at either end of the rod. If the current is pa.s.sed through the contact S_{1}, a current pa.s.ses through the left-hand solenoid, the core moves down, the rack on the core moves the pinion on the rod through a definite fraction of a complete revolution and this movement forces the creeper in one direction. Conversely, the pa.s.sing of the current through the solenoid at the other end of the threaded rod moves the creeper in the other direction. The distance which the iron rack on the end of the core is moved is determined carefully, so that the threaded rod is turned for each contact exactly the same fraction of a revolution. For actuating these solenoids, the 110-volt circuit is again used. The wire connections are shown in part in fig. 21, in which it is seen that the current pa.s.ses through the plunger-contact and through the pointer P to the silver plate S_{1} and then along the line G_{1} through 350 ohms wound about the left-hand solenoid back through a 600-ohm resistance to the main line. The use of the 110-volt current under such circ.u.mstances would normally produce a notable sparking effect on the pointer P, and to reduce this to a minimum there is a high resistance, amounting to 10,000 ohms on each side, shunted between the main line and the creeper connections. This shunt is shown in diagram in fig. 22. Thus there is never a complete open circuit and sparking is prevented.
THE CLOCK.
The clock requires winding every week and is so geared as to move the paper forward at a rate of 3 inches per hour. The contact-point for opening the circuit T on fig. 22 is likewise connected with one of the smaller wheels of the clock. This contact is made by tripping a little lever by means of a toothed wheel of phosphor-bronze.
INSTALLATION OF THE APPARATUS.
[Ill.u.s.tration: FIG. 23
Temperature recorder. The recorder with the coordinate paper in the lower box with a gla.s.s door. A curve representing the temperature difference between the ingoing and outgoing water is directly drawn on the coordinate paper. Above are three resistance boxes, and the switches for electrical connections are at the right. On the top shelf is the galvanometer, and immediately beneath, the plug resistance box for altering the value of certain shunts.]
[Ill.u.s.tration: FIG. 24.--Detailed wiring diagram showing all parts of recording apparatus, together with wiring to thermometers complete, including all previous figures.]
The whole apparatus is permanently and substantially installed on the north wall of the calorimeter laboratory. A photograph showing the various parts and their installation is given in fig. 23. On the top shelf is seen the galvanometer and on the lower shelf the recorder with its gla.s.s door in front and the coordinate paper dropping into the box below. The curve drawn on the coordinate paper is clearly shown. Above the recorder are the resistance-boxes, three in number, the lower one at the left being the resistance S_{1}, the upper one at the left being the resistance S', and the upper one at the right being the resistance Z_{1}. Immediately above the resistance-box Z_{1} is shown the plug resistance-box which controls on the one hand the resistance _r_ and on the other hand the resistance S, both of which are substantially altered when changing the apparatus to register from the 0 to 4 scale to the 0 to 8 scale. A detailed wiring diagram is given in fig. 24.
TEMPERATURE CONTROL OF THE INGOING AIR.
[Ill.u.s.tration: FIG. 25.--Section of calorimeter walls and part of ventilating air-circuit, showing part of pipes for ingoing air and outgoing air. On the ingoing air-pipe at the right is the lamp for heating the ingoing air. Just above it, H is the quick-throw valve for shutting off the tension equalizer IJ. I is the copper portion of the tension equalizer, while J is the rubber diaphragm; K, the pet-c.o.c.k for admitting oxygen; F, E, G, the lead pipe conducting the cold water for the ingoing air; and C, the hair-felt insulation. N, N are bra.s.s ferules soldered into the copper and zinc walls through which air-pipes pa.s.s; M, a rubber stopper for insulating the air-pipe from the calorimeter; O, the thermal junctions for indicating differences of temperature of ingoing and outgoing air and U, the connection to the outside; QQ, exits for the air-pipes from the box in which thermal junctions are placed; P, the dividing plate separating the ingoing and outgoing air; R, the section of piping conducting the air inside the calorimeter; S, a section of piping through which the air pa.s.ses from the calorimeter; A, a section of the copper wall; Y, a bolt fastening the copper wall to the 2-1/2 inch angle W; B, a portion of zinc wall; C, hair-felt lining of asbestos wall D; T-J, a thermal junction in the walls.]
In pa.s.sing the current of air through the calorimeter, temperature conditions may easily be such that the air entering is warmer than the outcoming air, in which case heat will be imparted to the calorimeter, or the reverse conditions may obtain and then heat will be brought away.
To avoid this difficulty, arrangements are made for arbitrarily controlling the temperature of the air as it enters the calorimeter.
This temperature control is based upon the fact that the air leaving the chamber is caused to pa.s.s over the ends of a series of thermal junctions shown as O in fig. 25. These thermal junctions have one terminal in the outgoing air and the other in the ingoing air, and consequently any difference in the temperature of the two air-currents is instantly detected by connecting the circuit with the galvanometer. Formerly the temperature control was made a varying one, by providing for either cooling or heating the ingoing air as the situation called for. The heating was done by pa.s.sing the current through an electric lamp placed in the cross immediately below the tension equalizer J. Cooling was effected by means of a current of water through the lead pipe E closely wrapped around the air-pipe, water entering at F and leaving at G. This lead pipe is insulated by hair-felt pipe-covering, C. More recently, we have adopted the procedure of pa.s.sing a continuous current of water, usually at a very slow rate, through the lead pipe E and always heating the air somewhat by means of the lamp, the exact temperature control being obtained by varying the heating effect of the lamp itself. This has been found much more satisfactory than by alternating from the cooling system to the heating system. In the case of the air-current, however, it is unnecessary to have the drop-sight feed-valve as used for the wall control, shown in fig. 13.