Magnetic compass 50 36. Magnet, swing to the left 50 37. Indicating direction of current 51 38. The bridge of the detector 52 39. Details of detector 53 40. Cross-section of detector 54 41. Acid voltameter 56 42. Copper voltameter 56 43. Two-pole switch 66 44. Double-pole switch 66 45. Sliding switch 67 46. Rheostat form of switch 68 47. Reversing switch 69 48. Push button 70 49. Electric bell 71 50. Armature of electric bell 72 51. Vertical section of annunciator 72 52. Front view of annunciator 72 53. Horizontal section of annunciator 72 54. Front plate of annunciator 72 55. Alarm switch on window 76 56. Burglar alarm on window 76 57. Burglar alarm contact 77 58. Neutral position of contact 78 59. Circuiting for electric bell 79 60. Annunciators in circuit 80 61. Wiring system for a house 80 62. Accumulator grids 83 63. Assemblage of accumulator grids 85 64. Connecting up storage battery in series 87 65. Parallel series 88 66. Charging circuit 88 67. Telegraph sending key 91 68. Telegraph sounder 92 69. A telegraph circuit 94 70. Induction coil and circuit 99 71. Illustrating elasticity 100 72. Condenser 101 73. High-tension circuit 102 74. Current interrupter 103 75. Wireless-telegraphy coherer 105 76. Wireless sending-apparatus 107 77.