Dobereiner’s traids When elements are arranged in the. Order of increasing atomic masses, group of three elements known as traids, having similar properties are obtained. The middle element of the traid being equal to the arithmetic mean of the atomic masses of the other two elements. Following are the examples. 1. The alkali Metal group Elements Symbols Atomic masses Lithium Li 7 Sodium Na 23 Potassium K 39 2. Alkaline Earth metal group Elements Symbols Atomic masses Calcium Ca 40 Strontium Sr 88 Barium Ba 137 3. The halogen group Elements Elements Symbols Atomic masses Chlorine Cl 35.5 Bromine
Projectors are basically used for presenting our ideas among others. In starting Projectors are mainly used in educational fields. Today we use projectors in educational fields as well as in business fields. In this article, I will show the projectors used in past that is Overhead Projectors. This topic is also listed in HP board computer science syllabus. Let's start with his definition. What is Overhead projector? Overhead projectors or OHP's are the type of projector used for project images on large flat wall. It has a flat transparent top and a light source beneath transparent top and reflecting mirror and a fan to cool it. Above the box there is a long arm that has a mirror to catch and redirect the light towards the screen. Uses of Overhead Projectors OHP is one of the most frequently used tool in education and training. Today, we use digital projectors in the place of OHP which is smaller in size as compared to OHP. Advantages of OHP 1. OHP's not much g