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Figure 39 is a diagram of a carbon-arc lamp. Two carbon rods are represented by two rectangles with an additional triangular point at one end. These rods are arranged horizontally with their pointed ends facing towards each other but with a noticeable space between the points. Above the two rods a circle is used to represent a high-voltage generator. Two lines are used to represent the electrical connections between the rods and the generator. Starting at the generator, one line leaves the right-hand side of the generator and connects to the right-hand end of the right-hand rod. The other line leaves the left-hand end of the left-hand rod and connects to the left-hand side of the generator. Between the pointed ends of the rods, a zig-zag line is drawn in an arc to represent a bright spark between the pointed ends of the rods. Overall the diagram shows a continuous electrical circuit going from the generator to the right-hand rod, through the right-hand rod, across the gap between the rods, through the left-hand rod and back to the generator.