ELECTRIC wave detectors, the most delicate part of the wireless receiving set, are the subject of much argument as to their relative sensitiveness and relative merits. There are a number of different types known, which may be arranged as follows in order of merit, according to most authorities: (1) Electrolytic; (2) Peroxide of Lead; (3) Perikon (Chalco-pyrites and zincite); (4) Ferron (Iron pyrites); (5) Silicon; (6) Molybdenite (Molybdenum Disulphide); (7) Galena (Lead Sulphide); (8) Carborundum (Artificial Silicon Carbide) . The Electrolytic Detector, perhaps the most widely known type, appears in many forms, the most frequently used of which is that in which the point of a very fine silver plated platinum wire, about 0.001 millimeter in diameter, called a Wollaston wire, is immersed in a nitric acid solution contained in a small graphite-carbon cup. It is necessary to use a battery and a non-inductive rheostat capable of very fine adjustment, called a potentiometer, in connection with this detector. The battery polarizes the electrolytic cell, that is, the fine platinum wire is. covered with tiny bubbles of oxygen and then the resistance rises so high that the current is nearly reduced to zero. If then an electric wave fall on the aerial connected to the electrolytic detector, it suddenly reduces the resistance of the cell. A telephone receiver of great sensitiveness being connected in series with the cell and potentiometer, sound signals can be heard in the telephone. For general and long distance work, this detector cannot be excelled except in localities where high powered interference is very frequent, which causes the platinum wire to be dissolved very rapidly and a detector working on different principles must be substituted. Substitutes will be mentioned later. The Lead Peroxide Detector consists essentially of a pellet of lead peroxide clamped between two surfaces, one of lead and one of platinum. A battery, potentiometer and telephone receiver are connected with the detector in the same manner as with the electrolytic detector. The lead peroxide detector works on a new principle; although no acids or liquids of any kind are employed, its action is electrolytic. When lead peroxide, lead and platinum are brought in contact as described, it has long been known that lead will be deposited on the platinum surface. When a battery is connected up properly with the detector, it opposes this action and causes the lead to be deposited upon the lead surface. When an electric wave falls upon the wires connected with the detector, a sound in the telephone is heard because of the change of resistance brought about by the passing of a wave. The Perikon Detector contact is between crystals of chalco-pyrites and zincite; in the Ferron Detector between a crystal of iron pyrites and a metal point; in the Silicon Detector between the element silicon and a wire; in the Molybdenite Detector between a crystal of molybdenum disulphide and a metallic surface; in the Galena detector between a-crystal of lead sulphide and a very fine wire; in the Carborundum Detector between a crystal of carborundum or silicon carbide and a metallic surface. The Silicon Detector retains its adjustment even when very near to a very high powered sending apparatus, and is therefore to be recommended where interference is very severe. If a piece of 99/100 per cent pure silicon can be procured, nothing better can be desired. The Carborundum Detector, even though subjected to somewhat severe blows, often retains its adjustment for' months at a time, which ca uses it to be the favorite detector with the amateur. However, it requires the use of a battery and potentiometer for its efficient use in long distance work. The green crystals are more sensitive than the blue or purple ones. The side of the crystal connected to the ground should be covered with tin foil or soldered in a cup. A New York firm has put on the market a promising new detector for which broad claims are made. This detector is of the vacuum bulb type and very similar to the “Audion” of Dr. Lee De Forest, which is not within the reach of the average experimentalist owing to its high cost. This detector consists of an electric light filament, a grid and a plate sealed in a highly exhausted bulb. A battery to light the filament and another battery and a rheostat capable of very fine adjustment are necessary for the operation of this detector. When the current to light the filament is turned on, the detector is ready for business, requiring no adjustments of crystals and points or Wollaston wire and acid. This detector can be used to receive from all systems of transmission; singing spark, quenched spark, arc sets, telephone sets, in fact it will detect any wireless wave from whatever source it is produced. The makers of this detector claim it to be more sensitive than the electrolytic, which will be a decided advance in wireless receiving apparatus. The wireless operator should not content himself with one detector, but should have several ready for use and a switch with which to connect up t.he desired one. A device of some kind should be used to ascertain whether or not the. detector of a wireless set is in a receptive condition. Several devices of this class are quite well known, but a short description will not be amiss. If a key sounder and battery are connected up neilr the detector, a sound is heard in the detector when the key is raised, if the detector is in working order. This method requires the constant operation of the key during the test or search for a sensitive area. But the author has found that if an ordinary electric bell with the gong removed, or a buzzer, is connected with a switch and battery, and two or three feet of copper wire be connected to the interpreter, the arrangement is .much more convenient and efficient. The short length of wire acts as an antenna, and owing to the speed of the vibrator, a continuous musical note is heard in the receiver when the detector is working properly. A variable condenser of good capacity should be shunted around the detector, enabling weak signals to be strengthened or cut out as desired. While transmitting, the detector should be short circuited by a switch to prevent its being burned out, or the wire in the telephone fused. When not in use, the detector should be protected from dust and moisture. The crystals of the thermo-electric type of detector should be touched with the hands as little as possible, for the oil secreted from the skin causes a marked decrease in the sensitiveness.