This thermometer is based upon the fact that the electrical resistance of a metallic wire increases with increase in temperature. It is seen that the resistance of a platinum wire increases uniformly with temperature. the additional advantage of platinum wire is that platinum does not chemically react with another substances and it melting point is quite high.<br />Construction:- in this thermometer a thin wire of pure platinum is doubled ( double wire becomes non inductive) and wound on a mica strip. The strip is placed at the bottom of a silica tube carrying an ebonite cap at its open end. The end of the platinum wire are joined to thick copper leads whose other ends are connected to the terminals PP fitted on the ebonite cap. To eliminate the effect of the resistance of copper lead , there are two other identical leads whose lower ends are joined together and the upper ends are connected to two other terminals CC. these leads are called compensating leads. <br />Trisha Banerjee @ 2010<br />
Advantages of platinum resistance thermometer:-<br /> a) it use is easier than that of a gas thermometer .<br /> b) it gives a precise measurement of a temperature because the resistance of the wire can be measured with high accuracy.<br /> c) it has a wide range from – 200 to 1200 degrees Celsius.<br /> d) it is quite sensitive . Its sensitivity is .01 degree up to 600 degree.<br /> e) it is free from change of zero as pure and annealed platinum wire has always the same resistance at the same temperature.<br /> f) the thermometer, once calibrated with standard hydrogen thermometer , can be used as a reliable standard thermometer.<br />Trisha Banerjee @ 2010<br />
Disadvantage of platinum resistance thermometer:-<br /> a) this thermometer has a large thermal capacity so that it takes appreciable heat from the experimental body. Hence the measured temperature is slightly lower than the actual temperature of the body.<br /> b)The tube of the thermometer has a low thermal conductivity . Therefore it takes time to acquire the temperature of the experimental body. moreover , some time is taken in the measuring resistance by the Wheatstone bridge . Hence temperature cannot be measured quickly.<br /> c) through the melting point of platinum is 1800 degree Celsius yet temperature higher than 1200 cannot be measured by the platinum thermometer . The reason is a that at higher temperatures the platinum begins to vaporize, resulting in considerable change in resistance of the wire.<br />Trisha Banerjee @ 2010<br />
The whole experiment is performed in three parts.<br />1) determination of resistance per unit length(p) of the bridge wire.<br /> a) Make the connections as shown in fig. above in which standard decimal resistance box is connected in the left outer gap and a thick copper strip in the right outer gap. Two nearly equal resistance P and Q are also connected in the inner gaps of the bridge.<br /> b) Insert a plug in a key , put the jockey on one end of the wire and note the direction of deflection . Now put the jockey on the other end of the wire ad again note direction of the deflection . If the directions on two cases are opposite connections may be assumed to correct.<br /> c) Take out a suitable resistance from the decimal resistance box and find the null point ( no deflection ) in galvanometer on the wire. Note down this balancing length l2’’ from the left end of the bridge wire.<br /> d) now interchange the positions of the copper strip and the decimal resistance box and again find the null point . Note down the balancing length l2’’ from the left end of the wire.<br /> e) Change the resistance r from the decimal resistance box and repeat above steps . Take atleast5 steps . Calculate the value of p .<br />Trisha Banerjee @ 2010<br />
2) Determination of the resistance R1 ( at room) and R2 at (100c ) of platinum resistance thermometer:-<br />Connections:- <br /> a) the ratio arms P and Q can also be obtained by connecting the lower terminals of the rheostat to the points M and N , its upper terminal ( sliding contact ) to the point C. the contact is adjusted.<br /> b) Now interchange the positions of the PP and CC leads connections to the outer gaps) and for the same resistance (Y) of decimal resistance box , find the position of the balancing point again . Note down this balancing length l2 from the left end of the bridge wire.<br />Go on changing the resistance (Y) from the decimal resistance box and for each value , take various sets of l1 and l2 . Calculate the resistance R1 by using R1 = Y + p( l2 – l1 )<br />Now place the bulb of the platinum resistance thermometer in a steam bath . Also introduce some resistance (Y’) in the decimal resistance box . When this thermometer attains the steady temperature of the bath , find the position of the balance point on the wire . Note down the balancing length l1’.<br />Similarly determine the balancing length l2’ after interchanging the position of the leads of outer gaps . Note down the temperature (t2) of the steam bath by a mercury thermometer and then calculate the value of R2 by using the formula.<br />Trisha Banerjee @ 2010<br />
The standard result i.e. The temperature coefficient of resistance for platinum = 37.0 X 10(-4) per degree Celsius.<br />Trisha Banerjee @ 2010<br />
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