Educational screen & audio-visual guide (c1956-1971])

use in a schoolroom is involved. In seeking to produce inexpensive television receivers, manufacturers have often employed selenium or silicon rectifiers to provide the high-voltage direct current needed in the tube circuits. A much better design from a safety standpoint uses a transformer and vacuum-tube or solid-state rectifier to get the direct-current electricity needed. The transformer gives the designer a means of isolating the chassis from the power line, and in this way he has a chance to keep leakage current much smaller than could occur with the selenium or silicon rectifiers alone. With many receivers, when this latter type of design is employed, the whole metal chassis may be at full line (120-volt) potential or not, depending only on the position of the plug in the wall outlet (whether a given prong of the plug happens to be at the top or at the bottom of the wall socket or receptacle, or at left or right, depending on the type of receptacle). The type of television receiver just described that has the so-called "hot-chassis" should not be used in the schoolroom, even though the design is such that the chassis seems completely shielded from exploring fingers, and even if it has U.L. approval. TV receivers that have rectifier systems using transformers are the kind to buy or rent. Particularly objectionable from a safety standpoint are metal cabinets, and this design should be avoided not only in any set used in the home, but especially in any set purchased for use in a schoolroom. No television receiving set employing a metal cabinet should be used in a schoolroom, even though the set may be entirely safe electrically when new. It is possible that unforeseen hazards may be created in use which could make the whole cabinet electrically "live"; if that should happen, the hazard to students and the teacher would be very great. A loose, frayed wire touching the inside of the cabinet or a very slight mistake in manufacture, or introduced in servicing the set could result in such a condition. This type of accident has occurred and has caused deaths of children. The cabinet of any schoolroom receiver should be well constructed and stoutly built. A weak cabinet presents a special hazard in circumstances when the receiver may be accidentally dropped or tipped over and the picture tube broken. Picture tubes are highly evacuated, and the exterior of a large tube therefore has tons of atmospheric pressure on it at all times. If the glass envelope should be broken and collapse, pieces of glass will not only be projected inwardly by For Educators Schoolmen will frequently find items of interest in Consumer Bulletin. Its editors report that mam/ schools across the nation— both secondartf schools and colleges— tise the publication regularly in class work. the external pressure, but may be propelled in any direction throughout the schoolroom with great force if the cabinet is not strong enough to contain them. So-called implosions of picture tubes are not uncommon. They may stem from a minor accident or from no cause at all. Thus there is no doubt that a strong well-built wooden cabinet should be present on any television set, particularly the large ones such as are likely to be used in a schoolroom. There is reason to believe that those new model 19 and 23-inch picture tubes which have the safety plate bonded directly to the face of the picture tube are less subject to implosion than those in which the safety shield is a separate piece of heavy glass or plastic. Any cart or stand upon which the receiver is placed so that it can be moved about on wheels or casters must be a rugged one, and the legs must be set far apart, so that any tendency of the set or the stand to tip over vWll be at a minimum. One must always allow for the possibility of an accident or horseplay in the schoolroom, or a set being moved across a wire or cable, a raised board, doorsill or threshold, or the edge of a rug; any such occunence might result in a strong tipping force being applied to the receiver. A metal stand, properly safeguarded, may be satisfactory, but from the standpoint of electrical safety, a stand made of wood would be best, and is recommended. ( One fatal accident to a child occurred in a home because a metal supporting stand formed part of the conducting path from the live television set chassis to the child's body, which at another point was in contact with a metal kitchen cabinet.) The lightweight parallel-type line cord with which TV sets are commonly equipped (a kind of wire that looks like ordinary lamp cord ) is satisfactory for home use, but is not recommended for use in a set placed in a schoolroom. For such use, a serviceman should remove the parallel-wire cord and install a length erf type SJ or SJT wire; these are the kinds of wire ir which the jacket or coating is strong enough so tha' it will stand some trampling or abuse by school chil dren without hazard. Wire of this grade will give longer life and reasonable freedom from fire or shock but if the wire or the connection either at the set o the connecting plug shows the slightest sign of dete rioration, it should at once be replaced by the school electrician or by a television repainnan. Finally, even though you may follow all of the fon going suggestions, it would be a wise precaution have any receiver looked over carefully and checki for electrical safety by a competent serviceman least twice yearly, or at the beginning of each sch()( term. The fact that a television receiver seems to wor satisfactorily provides no guarantee that it is safe f'l use. There is a "first time" for every electrical appl ance, and especially where children are concenie( the first small sliock or even a slight tingle calls /< correction at once. It is never wise to take chanci where electrical appliances are concerned, particular! those that present the high-voltage hazard and th degree of accessibility to children that goes with ever television set. 226 Educational Screen and Audiovisual Guide — May, 196' *