158 THE COLLIERY GUARDIAN July 28, 1916. Table II. ignet :ing res. 25 volts. 15 volts. 10 volts. 1 5 volts. br. ; ro df-in- liting ies. cells cause s & of nu hms. work Ampe: cur- peres. . to re- mini- hms. cur- peres. to re- mini- hms. cur- peres. to re- 1 mini- ■ hms. cur- peres. to re- in ini- hms. ■> of se at igr Henr 1° a ?! pi pC3 P® esistance coils. O'. Inimum current. aximum rent. Am asistance reduce to mum. O aximum rent. Am Fsistance duce to mum. O aximum rent. Am asistance duce to mum. 0 aximum rent. Am esistance duce to mum. O [inimum current i Amperes o-efficient duction current. Wet Lech required ignition. Pi ft P3 £ ft a ' O (L) ; (2.) (3-) (4.) (5.) (6.) (7.) (8) (9.) * (10.) (11) (12) i (13) 1 9'8 I 0’010 077 1,770 0-61 - 1,01 0 0-52 720 0 38 450 i 0’20 0-52 2 2 47 ! 0'012 i 0-35 1,555 023 875 0-17 610 Oil 1 300 i on 109 ! 4 3 48 0-042 ! 0-35 476 0-23 247 0-17 156 0-11 70 ; 0’12 0’84 4 4 48’2 0-004 ' 0’35 3,200 0’23 1,880 0-17 1,300 ! 0-11 770 0-10 1-20 4 5 99 0 008 0-20 2,485 0’12 1,350 009 915 0'05 488 0T2 1-00 9 G 99’5 0-010 0-20 1,530 0-12 845 0’09 570 ; 0-05 275 0-05 2-50 4 7 100 0-018 0’20 1,070 0’12 570 0-09 360 ' 0’05 170 0-05 2-50 4 8 102 0’039 0T9 488 0-12 240 0-09 138 0-05 35 o-io 1-20 8 9 250 0-005 009 3,155 0’05 1,825 0’04 1,170 0-02 522 0'05 2-50 10 10 510 0-006 004 2,310 0-02 1,100 0-015 635 . o-oi 150 * — — 11 515 0-015 0‘04 770 0-02 240 0015 12 i — , * — — * These two relays, of different patterns, had parallel short-circuited windings on the magnets. No ignition could be obtained with a battery of dry cells at 25 volts. less than that which occurs at the bell; possibly the flashes at the bell may cause ignition more readily, owing to their greater frequency in the same place. Safety. The degree of ‘ ‘ safety ” of a bell can be measured by its liability of causing, through self-induction, a dan- gerous break-flash either at the trembler contacts!or at the signalling point, the flashes that occur at the trembler contacts having the same nature as break-flashes at the signal wires, though their duration is short compared with the period of the bell. The duration of any break-flash depends upon the voltage of the circuit and the voltage added by self- induction. This inductance voltage is equal to the pro- duct of the coefficient of self-induction L, and the rate of change di/dt, of the current in the circuit. At the moment of break of circuit all the battery voltage can be regarded as absorbed in the resistance of the circuit, and does not materially affect the initial value of the inductance voltage. As soon as the current begins to fall (owing to the circuit being broken), the self-induc- tion voltage develops, rapidly reaches a maximum, and then falls. While this occurs, the battery voltage across Fig. 2. the break of circuit is increasing until, when the current is zero, the full battery voltage is established. This is well shown in fig. 2, which is an oscillogram of the voltage across the trembler contacts of a ringing bell. When the circuit voltage is low and the inductance high, as in a bell signalling system, the minimum igniting current for the break-flash is inversely propor- tional to the inductance, so that the product Lz is con- stant, as is shown by the following series of determina- tions of igniting currents with circuits having different inductances. Different values for the inductance were obtained by including in the circuit a signalling bell, the magnet of which was wound with different numbers of turns of wire. The flash was produced mechanically by a rapid break of circuit:— Table III. Inductance Igniting current (i) (L). Henry. (at 25 volts.) ‘ Ampere. Li. 0'27 0-82 0’220 0-47 0-45 0'212 0’70 0-26 0-182 0’90 0’20 0’180 104 0-17 0T77 1-18 0155 0-183 1’27 0’145 0’184 1-31 0-13 0-170 1-60 o-u 0176 2-00 0’09 0-180 Since the mechanical rate of break was the same, throughout this series of determinations, it 'follows that di/dt was proportional to the current broken, so that, since Lz was found to be constant, the inductance voltage, ~Ldi/dt, was also constant. This point was verified by taking oscillograms of the current at break from which the voltage and the rate of change of the •current could be measured. It may be said, therefore, that ignition by a rapid break-flash at a low circuit voltage depends on the inductance voltage at which the flash is formed, and the igniting power of the flash is proportional to the product Lz When the break of circuit is made slowly, the igniting power of the flash has been found to depend upon its energy, |Lz2. There are thus two limiting con- ditions for the igniting power of the flash; at the one the inductance voltage is of importance, at the other the energy. For any given gaseous mixture there is a range of rapidity of break over which the two types of ignition blend, so that under certain conditions the igniting power of the flash may be proportional neither directly to z nor to i2, but to some intermediate value of i. Thus in the previous report, the relation between L and z was determined when L varied between 0-008 henry and 0-09 henry, and the current was at 90 volts pressure. In this case Lz1 4 = k, so that the igniting power of the break-flash fell between the conditions under which it would be proportional to the inductance voltage (Ldi/dt), and thus proportional to Lz, and the conditions under which it would be proportional to its energy (JLz2). The inductance voltage Es = Lidi/dt = kLd, where Zeis a constant. Since i= E/r, therefore Es = fcL.E/r. The inductance voltage is thus inversely proportional to the resistance of the circuit when, as can be considered roughly the case in practice, the battery, voltage and inductance are constant. In so far, therefore, as the ignition of inflammable mixtures is dependent on the product Lz, ignition can be prevented by the use of high resistance windings and high resistance batteries. The energy of the break-flash, ^Lz2 or JLE2/r2, is even more dependent on the circuit resistance, being inversely proportional to its square. Mechanical and Electrical Efficiency. The current at which ringing just ceases depends upon the adjustment of the mechanical tensions (of the armature spring and, in most bells, of the lighter spring control derived from the contact strip on the armature), and upon the length of air gap, as well as upon the electro-magnetic pull. Examples of the differences in the minimum currents of bells as set by the makers have been given in Table I. From experiments made as to the most efficient setting for bells, it would appear to be advisable to use as light a spring on the armature as will suffice to maintain good contact at the trembler in readiness to ring and to return the armature to its first position after the ringing blow has been given. Any slight sacrifice in the rate of ringing or loudness of sound that may be caused by using a light spring on the armature is more than compensated for by the fact that the bell can be actuated by a small current. Electrically the chief effect produced by a current in passing round the bell coils is the magnetisation of the cores. This magnetisation grows with the current, and is proportional to -the ampere turns of the windings. With a given battery voltage, the ringing power of a bell varies inversely with the resistance of the circuit. It is not possible, however, to work with a very low resistance in circuit, since the current is limited both by the kind of battery used and by considerations of danger due to sparking at the bell or point of signalling. Safety Combined with Efficiency. A bell with many turns of wire gives the greatest ringing power per igniting current ampere, is, in fact, the “ safest-efficient ” bell (apart from the use of special devices), on a circuit of given voltage and constant resistance. To test this a series of experiments was made to see how the ringing power and the minimum igniting current were affected by the number of turns of wire in the magnet windings. The bell had a single bobbin wound with 32-gauge silk-covered copper wire, 112 turns per layer. To obtain a measure of the ringing power a method similar to the photometric method of comparing the intensities of lights was used. A bell of the same pattern and tone of gong as the experimental bell was kept ringing under constant conditions, and the experi- mental bell moved until it was judged by observers at a fixed point to have the same intensity of sound as the standard. The results were :— Layers of wire Igniting current Relative sounding power (S) S/i. on magnet. (i). Ampere. Measured at i. 4 0-82 0’32 0’39 8 0-45 0 63 1-40 12 0’26 0*63 242 16 0-20 0-90 4-50 18 0’17 0’96 5’65 20 0’155 0-90 5’80 22 0145 0-90 6’20 24 0’13 1-00 7’70 28 0’11 0-96 8’72 32 009 0-63 7-00 There is thus for every bell and size of winding wire a definite number of turns at which efficiency of ringing combined with “safety” of the self-induction break- flash reaches a maximum. With regard to the diameter of the core, it would appear that, apart from changes in the inductance due to changes in the position of the armature, mining bells and relays have iron cores of such small cross-section that the iron becomes saturated at quite low currents, and the consequent fall in permeability as the current is increased reduces the coefficient of self-induction. The values of the coefficient for bell No. 1 at different currents, the air-gap being kept constant, are shown plotted as a curve in fig. 3, and for comparison the curve representing the change of permeability, of iron with magnetic field H (= 4tf/10 times the ampere turns per centimetre of magnetic circuit). The two curves are identical in form, and it is clear that the magnetisation of the iron cores of mining bells and relays dominates their inductance, and that the reluctance of the air-gap is not the controlling factor of the magnetic circuit. From this it can be concluded that, probably for economy in winding, the iron magnet cores of mining bells (in particular) and relays are too small, being saturated too easily. It would seem that, in order to obtain the maximum efficiency the diameter of the core should be between 0-5 and 0-6 times that of the bobbin. It has been assumed, however, that the mean value of /z is not affected by change of area of the core, whereas in reality /x increases with the area. Further, the calculations make no allowance for eddy currents in the core. Experi- ments show that these factors combine to reduce the diameter of core requisite to give the maximuni efficiency; and, taking all the factors into consideration, the general conclusion can be drawn that the most Fig 3. efficient mining bells should have bobbins wound with from 25 to 30 layers of fine wire, and that the diameter of the core should be between 0-4 and 0-5 times that of the bobbin. Methods of Rendering Bells and Relays Safe. It has been shown that the dangerous nature of the break-flash that occurs when a bell or relay is included in 'the electrical circuit is ill part due to the occurrence of self-induction which, when the circuit is broken, pro- duces momentarily an abnormal voltage. Suitable methods of rendering bells and relays safe, therefore, are such as aim at minimising the effects of the highly self-inductive electromagnetic windings which are the essential features of bells and relays. It is possible, however, to construct good ringing bells and efficient relays without having recourse to special devices for overcoming the effects of self-induction. The chief factor in rendering the break-flash at the signal wires dangerous is the amount of current available. Given a definite battery power that is not to be exceeded, the resistance of the bell or relay can be so proportioned that the maximum current obtainable on short-circuit does not exceed the “ minimum igniting current ” for the system. High Resistance Winding.—The* required resistance can be obtained either by means of a non-inductively wound coil in series with the magnet coils, or by winding the coils with a high resistance wire. The latter is the simplest method of dealing with the problem. Its efficacy is illustrated by bell No. 17 (Table I.); this bell, which was of type B, differed from ordinary bells of that ■type only in having magnet windings of brass (25-gauge) instead of copper wire. The resistivity of brass being about six times that of copper, the magnet coils, though comparatively few in number (and therefore not exces- sively self-inductive), were of - high resistance. The maximum current obtainable from a battery of wet Leclanche cells at 25 volts when this bell was included in the circuit was insufficient to give a break-flash that could ignite the most sensitive methane air mixture; while the battery efficiency was good, over 200 ohms additional resistance being required before the bell ceased to ring. There are several ways of overcoming the effects of self-induction. The inductance voltage, which may render the break-flash dangerous, is derived from a sudden change in the number of magnetic lines of force in the windings when the circuit is broken, and, as pre- viously mentioned, is proportional to their rate of change. Any device, therefore, which retards the change of magnetism on break of circuit lessens the break-flash voltage. In this respect parallel-winding, copper sleeves, shunt resistances, and tin-foil layers are identical in their action. Parallel Winding.—Two windings on the bobbins are carried in parallel throughout, only one winding is used as the exciting winding for the magnet, the other is short- circuited on itself. When a current is passed through the exciting winding a current is induced in the short- circuited winding, and when the main current is broken the induced current opposes the change of magnetism which causes the electro-motive force of self-induction of the exciting winding, so that the intensity of the break-flash is greatly reduced. As with ordinary single winding, the best diameter for the core of the magnet is between 0-4 and 0-5 times the diameter of the bobbin.